@arreally you really haven't seen a male birth in hentai before? From what I've seen typically the urethra is stretched out like crazy to pass the baby through the penis, but sometimes you see the baby come out of their ass too typically following their water breaking and the umbilical cord hanging out of the rectum... yeah the whole thing is quite silly and it's best not to think about the poop aspect. Silly fetish but it's fun to pretend :P