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1boy 1girl 2020s 2023 2d animated_skeleton artist_name artist_watermark blue_breasts blue_nipples blue_tongue bottom_sans breasts clothed clothed_male clothing cum cum_on_face digital_media duo ech7474 ectopenis female_sans genderswap genderswap_(mtf) genitals gloves hooded_jacket hoodie jacket licking licking_face licking_penis male male/female monster multiple_views nipples nude off_shoulder orange_penis paizuri penis purple_background sans_(undertale) sanster seme_gaster sex simple_background skeleton solid_color_background sweater tears top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster watermark white_gloves
1boy 1girl 2020s 2023 2d animated_skeleton artist_name artist_watermark blue_breasts blue_nipples blue_tongue bottom_sans breasts clothed clothed_male clothing cum cum_on_face digital_media duo ech7474 ectopenis female_sans genderswap genderswap_(mtf) genitals gloves hooded_jacket hoodie jacket licking licking_face licking_penis male male/female monster multiple_views nipples nude off_shoulder orange_penis paizuri penis purple_background sans_(undertale) sanster seme_gaster sex simple_background skeleton solid_color_background sweater tears top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster watermark white_gloves
2010s 2016 2boys 2d animated_skeleton arms_behind_back artist_name beoteono black_tentacles blue_blush blue_penis blush bodily_fluids bottom_sans buttercupsticks clothed clothing cussing digital_media dominant dominant_male drooling duo ectopenis feet_out_of_frame fluids grabbing_another's_chin grabbing_chin grey_background hand_under_chin hands_behind_back male male/male male_focus male_only maledom malesub monster one_eye_closed partially_colored penis precum profanity sans_(undertale) sanster seme_gaster simple_background skeleton soul soul_sex submissive submissive_male swearing tentacle tentacle_on_male tentacles tentacles_around_legs tentacles_on_male text top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2010s 2016 2boys 2d animated_skeleton arms_behind_back artist_name beoteono black_tentacles blue_blush blue_penis blush bodily_fluids bottom_sans buttercupsticks clothed clothing cussing digital_media dominant dominant_male drooling duo ectopenis feet_out_of_frame fluids grabbing_another's_chin grabbing_chin grey_background hand_under_chin hands_behind_back male male/male male_focus male_only maledom malesub monster one_eye_closed partially_colored penis precum profanity sans_(undertale) sanster seme_gaster simple_background skeleton soul soul_sex submissive submissive_male swearing tentacle tentacle_on_male tentacles tentacles_around_legs tentacles_on_male text top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2010s 2017 2d 4boys animated_skeleton blue_blush blue_tongue blush bottom_sans bukkake clothed clothing cum cum_in_mouth cum_on_tongue digital_media ectotongue fangs finger_in_mouth fontcest gangbang grillby grillsans hand implied_incest lowest-of-dirt male male/male male_focus male_only monster multiple_boys one_eye_closed orange_cum orange_penis papyrus_(undertale) papysans penis red_background sans_(undertale) sanster seme_gaster seme_grillby seme_papyrus sex skeleton solo_focus tongue top_gaster top_grillby top_papyrus uke_sans undead undertale undertale_(series) unseen_character unseen_male unseen_male_face upper_body video_game_character video_games w.d._gaster yaoi
2010s 2017 2d 4boys animated_skeleton blue_blush blue_tongue blush bottom_sans bukkake clothed clothing cum cum_in_mouth cum_on_tongue digital_media ectotongue fangs finger_in_mouth fontcest gangbang grillby grillsans hand implied_incest lowest-of-dirt male male/male male_focus male_only monster multiple_boys one_eye_closed orange_cum orange_penis papyrus_(undertale) papysans penis red_background sans_(undertale) sanster seme_gaster seme_grillby seme_papyrus sex skeleton solo_focus tongue top_gaster top_grillby top_papyrus uke_sans undead undertale undertale_(series) unseen_character unseen_male unseen_male_face upper_body video_game_character video_games w.d._gaster yaoi
1boy 1cuntboy 2010s 2017 2d ??? anal anal_fisting animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page comic_panel cuntboy digital_media double_penetration drooling ectopussy edit english_text fingers_in_pussy fisting fisting_pussy grabbing_mouth large_filesize long_image male male/cuntboy monochrome monster mouth_held_open multiple_hands nczhhdyb pants_pulled_down pants_pulling pussy pussy_juice sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus spoken_heart spoken_question_mark spread_anus spread_pussy tears text third-party_edit tongue tongue_out top_gaster uke_sans uncensored undead undertale undertale_(series) urethra vaginal vaginal_fisting video_games w.d._gaster wet_pussy yaoi
1boy 1cuntboy 2010s 2017 2d ??? anal anal_fisting animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page comic_panel cuntboy digital_media double_penetration drooling ectopussy edit english_text fingers_in_pussy fisting fisting_pussy grabbing_mouth large_filesize long_image male male/cuntboy monochrome monster mouth_held_open multiple_hands nczhhdyb pants_pulled_down pants_pulling pussy pussy_juice sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus spoken_heart spoken_question_mark spread_anus spread_pussy tears text third-party_edit tongue tongue_out top_gaster uke_sans uncensored undead undertale undertale_(series) urethra vaginal vaginal_fisting video_games w.d._gaster wet_pussy yaoi
1boy 1cuntboy 2010s 2017 2d ??? anal anal_fisting animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page comic_panel cuntboy digital_media double_penetration drooling ectopussy edit edited english_text fingers_in_pussy fisting fisting_pussy grabbing_mouth large_filesize long_image male male/cuntboy monochrome monster mouth_held_open multiple_hands nczhhdyb pants_pulled_down pants_pulling pussy pussy_juice sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus spoken_heart spoken_question_mark spread_anus spread_pussy tall_image tears text third-party_edit tongue tongue_out top_gaster uke_sans uncensored undead undertale undertale_(series) urethra vaginal vaginal_fisting video_games w.d._gaster wet_pussy yaoi
1boy 1cuntboy 2010s 2017 2d ??? anal anal_fisting animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page comic_panel cuntboy digital_media double_penetration drooling ectopussy edit edited english_text fingers_in_pussy fisting fisting_pussy grabbing_mouth large_filesize long_image male male/cuntboy monochrome monster mouth_held_open multiple_hands nczhhdyb pants_pulled_down pants_pulling pussy pussy_juice sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus spoken_heart spoken_question_mark spread_anus spread_pussy tall_image tears text third-party_edit tongue tongue_out top_gaster uke_sans uncensored undead undertale undertale_(series) urethra vaginal vaginal_fisting video_games w.d._gaster wet_pussy yaoi
2boys 2d ahegao animated_skeleton artist_request blue_tongue blush bottom_sans crying_with_eyes_open digital_media dominant_pov english_text fangs hands hands_behind_head male male/male male_ahegao male_focus male_only moan monster multiple_hands pleasure_face pov sans_(undertale) sanster seme_gaster skeleton solo_focus text tongue tongue_out top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2boys 2d ahegao animated_skeleton artist_request blue_tongue blush bottom_sans crying_with_eyes_open digital_media dominant_pov english_text fangs hands hands_behind_head male male/male male_ahegao male_focus male_only moan monster multiple_hands pleasure_face pov sans_(undertale) sanster seme_gaster skeleton solo_focus text tongue tongue_out top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
1boy 1cuntboy 2010s 2017 2d anal anal_fisting animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page comic_panel cuntboy double_penetration drooling ectopussy fisting fisting_pussy grabbing_mouth male male/cuntboy monochrome monster mouth_held_open multiple_hands nczhhdyb pussy sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus tears tongue tongue_out top_gaster uke_sans uncensored undead undertale undertale_(series) vaginal vaginal_fisting video_games w.d._gaster yaoi
1boy 1cuntboy 2010s 2017 2d anal anal_fisting animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page comic_panel cuntboy double_penetration drooling ectopussy fisting fisting_pussy grabbing_mouth male male/cuntboy monochrome monster mouth_held_open multiple_hands nczhhdyb pussy sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus tears tongue tongue_out top_gaster uke_sans uncensored undead undertale undertale_(series) vaginal vaginal_fisting video_games w.d._gaster yaoi
1boy 1cuntboy 2010s 2017 2d ??? animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page cuntboy ectopussy fingers_in_pussy male male/cuntboy monochrome monster multiple_hands nczhhdyb pants_pulled_down pants_pulling pussy pussy_juice sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus spoken_heart spoken_question_mark spread_anus spread_pussy top_gaster uke_sans uncensored undead undertale undertale_(series) urethra video_games w.d._gaster wet_pussy yaoi
1boy 1cuntboy 2010s 2017 2d ??? animated_skeleton anus black_and_white bottom_sans clitoris comic comic_page cuntboy ectopussy fingers_in_pussy male male/cuntboy monochrome monster multiple_hands nczhhdyb pants_pulled_down pants_pulling pussy pussy_juice sans_(undertale) sanster seme_gaster sequence sequential skeleton solo_focus spoken_heart spoken_question_mark spread_anus spread_pussy top_gaster uke_sans uncensored undead undertale undertale_(series) urethra video_games w.d._gaster wet_pussy yaoi
1:1 1:1_aspect_ratio 2boys 2d animated_skeleton blue_blush blue_tongue bottom_sans daron9 digital_media fur_trim implied_penetration implied_sex korean_text male male_focus male_only monster sans_(undertale) sanster seme_gaster skeleton solo_focus text tongue tongue_out top_gaster uke_sans undead undertale undertale_(series) unseen_male_face video_games w.d._gaster yaoi
1:1 1:1_aspect_ratio 2boys 2d animated_skeleton blue_blush blue_tongue bottom_sans daron9 digital_media fur_trim implied_penetration implied_sex korean_text male male_focus male_only monster sans_(undertale) sanster seme_gaster skeleton solo_focus text tongue tongue_out top_gaster uke_sans undead undertale undertale_(series) unseen_male_face video_games w.d._gaster yaoi
2015 2boys 2d animated_skeleton black_background blue_blush blush bottom_sans clothed digital_media duo groping hand hand_on_crotch hands male male_only monster multiple_hands sans_(undertale) sanster seme_gaster shirt_up skeleton solo_focus suckmytrombone sweater top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2015 2boys 2d animated_skeleton black_background blue_blush blush bottom_sans clothed digital_media duo groping hand hand_on_crotch hands male male_only monster multiple_hands sans_(undertale) sanster seme_gaster shirt_up skeleton solo_focus suckmytrombone sweater top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2020s 2022 2d animated_skeleton arm_grab bad_id bad_twitter_id black_and_white bottom_sans clothed covering_mouth digital_media fingering fingering_another fingering_partner grabbing_arm grabbing_hand hand hand_grab hand_on_mouth hands inactive_account male male_focus male_only monochrome monster monster_boy multiple_hands non-con non-consensual noncon nonconsensual pants_pull pants_pulled_down sans_(undertale) sanster seme_gaster shirt_lift shirt_pull shirt_up simple_background skeleton socks_on solo_focus top_gaster toubigaizi uke_sans undead undertale undertale_(series) undressing unseen_character unseen_male_face video_game_character video_games w.d._gaster white_hair yaoi
2020s 2022 2d animated_skeleton arm_grab bad_id bad_twitter_id black_and_white bottom_sans clothed covering_mouth digital_media fingering fingering_another fingering_partner grabbing_arm grabbing_hand hand hand_grab hand_on_mouth hands inactive_account male male_focus male_only monochrome monster monster_boy multiple_hands non-con non-consensual noncon nonconsensual pants_pull pants_pulled_down sans_(undertale) sanster seme_gaster shirt_lift shirt_pull shirt_up simple_background skeleton socks_on solo_focus top_gaster toubigaizi uke_sans undead undertale undertale_(series) undressing unseen_character unseen_male_face video_game_character video_games w.d._gaster white_hair yaoi
2boys 2d animated_skeleton bigger_dom bigger_dom_smaller_sub bigger_male bigger_penetrating bigger_penetrating_smaller blue_blush blue_tongue bottom_sans digital_media english_text from_behind heart-shaped_pupils heart_eyes height_difference humanoid humanoid_penetrating larger_male larger_penetrating larger_penetrating_smaller male male_focus male_only male_penetrating monochrome nude one_eye_closed penetration sans_(undertale) sanster seme_gaster sex sex_from_behind skeleton small_sub small_sub_big_dom smaller_penetrated smaller_sub smaller_sub_bigger_dom tesno204 text tongue tongue_out top_gaster topless topless_male uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2boys 2d animated_skeleton bigger_dom bigger_dom_smaller_sub bigger_male bigger_penetrating bigger_penetrating_smaller blue_blush blue_tongue bottom_sans digital_media english_text from_behind heart-shaped_pupils heart_eyes height_difference humanoid humanoid_penetrating larger_male larger_penetrating larger_penetrating_smaller male male_focus male_only male_penetrating monochrome nude one_eye_closed penetration sans_(undertale) sanster seme_gaster sex sex_from_behind skeleton small_sub small_sub_big_dom smaller_penetrated smaller_sub smaller_sub_bigger_dom tesno204 text tongue tongue_out top_gaster topless topless_male uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2boys 2d anal animated_skeleton bigger_dom bigger_dom_smaller_sub bottom_sans carrying carrying_partner carrying_position cussing digital_media digital_media_m dominant dominant_male domination duo ectopenis english_text gay in_heat licking_face male male/male male_focus male_only maledom malesub monster monster_boy penis profanity reverse_stand_and_carry_position reverse_suspended_congress sans_(undertale) sanster seme_gaster sex simple_background skeleton sketch small_sub small_sub_big_dom smaller_male smaller_sub smaller_sub_bigger_dom spoopy-gaster submissive submissive_male swearing tentacle tentacle_around_penis tentacle_on_male tentacle_sex text text_bubble top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2boys 2d anal animated_skeleton bigger_dom bigger_dom_smaller_sub bottom_sans carrying carrying_partner carrying_position cussing digital_media digital_media_m dominant dominant_male domination duo ectopenis english_text gay in_heat licking_face male male/male male_focus male_only maledom malesub monster monster_boy penis profanity reverse_stand_and_carry_position reverse_suspended_congress sans_(undertale) sanster seme_gaster sex simple_background skeleton sketch small_sub small_sub_big_dom smaller_male smaller_sub smaller_sub_bigger_dom spoopy-gaster submissive submissive_male swearing tentacle tentacle_around_penis tentacle_on_male tentacle_sex text text_bubble top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2022 2d animated_skeleton blue_blush blue_hoodie blue_jacket blush bottom_sans bound bound_arms chains clothed digital_media disembodied_hands dominant_pov drooling ectoplasm fingering fingering_another fingering_partner hooded_jacket hoodie jacket japanese_text leg_grab legs_apart logo_(pixiv_id_30462562) looking_at_viewer male monster monster_boy partially_clothed partially_nude potion pov restrained restraints sans_(undertale) sanster seme_gaster skeleton solo_focus submissive sweat text top_gaster uke_sans undead undertale undertale_(series) unseen_male unseen_male_face video_games w.d._gaster yaoi
2022 2d animated_skeleton blue_blush blue_hoodie blue_jacket blush bottom_sans bound bound_arms chains clothed digital_media disembodied_hands dominant_pov drooling ectoplasm fingering fingering_another fingering_partner hooded_jacket hoodie jacket japanese_text leg_grab legs_apart logo_(pixiv_id_30462562) looking_at_viewer male monster monster_boy partially_clothed partially_nude potion pov restrained restraints sans_(undertale) sanster seme_gaster skeleton solo_focus submissive sweat text top_gaster uke_sans undead undertale undertale_(series) unseen_male unseen_male_face video_games w.d._gaster yaoi
2020s 2022 2d animated_skeleton blue_blush blue_hoodie blue_jacket blue_tongue blush bottom_sans clothed crying digital_media duo hooded_jacket hoodie jacket japanese_text logo_(pixiv_id_30462562) male male_penetrating monster monster_boy penetration sans_(undertale) sanster seme_gaster sex skeleton submissive sweat tears text tongue top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2020s 2022 2d animated_skeleton blue_blush blue_hoodie blue_jacket blue_tongue blush bottom_sans clothed crying digital_media duo hooded_jacket hoodie jacket japanese_text logo_(pixiv_id_30462562) male male_penetrating monster monster_boy penetration sans_(undertale) sanster seme_gaster sex skeleton submissive sweat tears text tongue top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2017 2boys 2d animated_skeleton blue_blush blue_tongue blush bottom_sans crying crying_with_eyes_open digital_media male male_penetrating monster penetration penetration_from_behind pillow_grab sans_(undertale) sanster seme_gaster sex sex_from_behind sinfulsanspar skeleton sketch smiling tears tongue tongue_out top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2017 2boys 2d animated_skeleton blue_blush blue_tongue blush bottom_sans crying crying_with_eyes_open digital_media male male_penetrating monster penetration penetration_from_behind pillow_grab sans_(undertale) sanster seme_gaster sex sex_from_behind sinfulsanspar skeleton sketch smiling tears tongue tongue_out top_gaster uke_sans undead undertale undertale_(series) video_games w.d._gaster yaoi
2020s 2022 2boys 2d abosinsin animated_skeleton bad_id bad_twitter_id big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bigger_male bigger_penetrating bigger_penetrating_smaller blue_blush blue_penis blush bottom_sans commission dialogue digital_media duo ectopenis ectoplasm edging english_text face_grab gay gay_sex grabbing_face grabbing_penis height_difference holding_face inactive_account larger_penetrating male male/male male_only male_penetrated male_penetrating male_penetrating_male monster orgasm_denial partially_colored penetration penis penis_grab purple_blush purple_penis sans_(undertale) sanster seme_gaster sitting skeleton small_sub small_sub_big_dom smaller_male smaller_penetrated smaller_sub smaller_sub_bigger_dom text third-party_source top_gaster twitter_username uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster white_background yaoi
2020s 2022 2boys 2d abosinsin animated_skeleton bad_id bad_twitter_id big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bigger_male bigger_penetrating bigger_penetrating_smaller blue_blush blue_penis blush bottom_sans commission dialogue digital_media duo ectopenis ectoplasm edging english_text face_grab gay gay_sex grabbing_face grabbing_penis height_difference holding_face inactive_account larger_penetrating male male/male male_only male_penetrated male_penetrating male_penetrating_male monster orgasm_denial partially_colored penetration penis penis_grab purple_blush purple_penis sans_(undertale) sanster seme_gaster sitting skeleton small_sub small_sub_big_dom smaller_male smaller_penetrated smaller_sub smaller_sub_bigger_dom text third-party_source top_gaster twitter_username uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster white_background yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bodily_fluids bottom_sans clothed clothed/nude completely_nude crying digital_media duo fingering fingering_another fingering_partner fluids fully_clothed height_difference humanoid male male/male male_only monochrome monster monster_boy nude pf-pro-fucker sans_(undertale) sanster seme_gaster size_difference skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom tears top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster white_background yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bodily_fluids bottom_sans clothed clothed/nude completely_nude crying digital_media duo fingering fingering_another fingering_partner fluids fully_clothed height_difference humanoid male male/male male_only monochrome monster monster_boy nude pf-pro-fucker sans_(undertale) sanster seme_gaster size_difference skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom tears top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster white_background yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub blush_lines bottom_sans digital_media duo height_difference humanoid male male/male monochrome monster monster_boy one_eye_closed pf-pro-fucker sans_(undertale) sanster seme_gaster skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom sweat top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub blush_lines bottom_sans digital_media duo height_difference humanoid male male/male monochrome monster monster_boy one_eye_closed pf-pro-fucker sans_(undertale) sanster seme_gaster skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom sweat top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bodily_fluids bottom_sans bottomless digital_media duo fingering fingering_another fingering_partner fluids height_difference humanoid labcoat male male/male monochrome monster monster_boy pf-pro-fucker sans_(undertale) sanster seme_gaster skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bodily_fluids bottom_sans bottomless digital_media duo fingering fingering_another fingering_partner fluids height_difference humanoid labcoat male male/male monochrome monster monster_boy pf-pro-fucker sans_(undertale) sanster seme_gaster skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bottom_sans clothed/nude digital_media duo ectoplasm fingering height_difference humanoid male male/male male_focus male_only monochrome monster monster_boy pf-pro-fucker sans_(undertale) sanster seme_gaster skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster yaoi
2010s 2018 2boys 2d animated_skeleton big_dom big_dom_small_sub bigger_dom bigger_dom_smaller_sub bottom_sans clothed/nude digital_media duo ectoplasm fingering height_difference humanoid male male/male male_focus male_only monochrome monster monster_boy pf-pro-fucker sans_(undertale) sanster seme_gaster skeleton small_sub small_sub_big_dom smaller_sub smaller_sub_bigger_dom top_gaster uke_sans undead undertale undertale_(series) video_game_character video_games w.d._gaster yaoi