Responses
It was written on the cover of chapter 3 that it is possible to artificially make a Nier male pregnant.
But I don't know why they made the human dad the one who's bearing the baby when it will be easier if the BoB dad is the one (since his body is designed to be able to bear a baby). Maybe himan's semen is too weak to impregnate a BoB? ╮(￣▽￣)╭
I'm having some issues with this manga's lore. I'm pretty sure (from the latest chapter especially) that "gender differentiation" refers to whether B.O.B. can bear children? Which doesn't actually make sense as "gender," they're all pretty obviously gendered as men, including Nishiki. "Secondary sex characteristic differentiation" would make more sense since it's biological sexual characteristics that are linked to reproduction. Gender isn't. Unless "gender differentiation" means that a pretty large number of beastpeople are trans? In which case they *would* choose/figure out their gender, and some B.O.B. who were gendered men *could* bear kids (which would be a pretty awesome and original setup). But they'd know from the start whether they could bear children because again, that's biological, nothing to do with gender.
I personally could have done without this altogether since I dislike mpreg in manga (to be clear I do not have any problem at all with trans men bearing kids though), but whatever. I like the mangaka, I read, unless it's omegaverse tagged with mpreg. And I really like the beastpeople aspect.
Leaving that aside. I'm going to have to assume that the Lycan dad is the one who's bearing the kid since the human dad is HUMAN and, as established in the first or second chapter, humans don't have "gender differentiation" in this world, only B.O.B. do.
Edit to add: I checked and the human dad said "I already entered the stable period" so unfortunately I think the mangaka might have f'ed up and forgotten their own lore