anal anal_sex anthro anus ass close-up doggy_position echidna erection furry hedgehog knuckles_the_echidna lying male mammal monotreme nude on_floor on_front on_top penetration penis presenting presenting_anus presenting_hindquarters quills sega sex simple_background sonic_the_hedgehog spread_legs spreading squish_(artist) testicles thick_thighs white_background yaoi