Just a flow of thoughts. Well, we dont need to associate the fact Birds poo in that only opening used for sex. There are tons of games and cartoons whose characters dont have even an anus. If we want realism in regard to anatomy, just make one opening (the cloaca: the anus that is also a vagina), then make it get aroused like a pussy, and make it cum, and that's all. No food leftovers. A clean hole used for sex and tons of pleasure
In MIM PUT, Peach is like that. She doesnt poo. She eats and her food turns into biological chemicals to be used by her own needs, and the rest becomes cum, sweat and 'tender female scents' - pherormones. She uses her anus as a hammerspace area. I find it interesting for inventory items, and it explains those games where the characters can have many objects getting carried without reason.
I respect other opinions and how people see anal as 'dirty', but really, is it so difficult to recreate the anus to be a clean opening? Even if me, you, someone else 'remembers' of poo, its not so hard to rethink and to notice those characters are fantasy beings. They're different. Their openings are special, seen in another angle, enticing for those who can explore it
Its all about redefinitions. A different way to see things. To deassociate 'anus' with 'dirty'. Maybe we can call / recall those openings as vagina holes, or special holes. Birds with strong and extremelly arousing holes, those they are ashamed of showing, but they know this is the way they can copulate.