What if Beastkin and Humans can have children it's just that they have a harder time conceiving(???? Is that the right word¿???)
It might also explain (what i assume( why there aren't a lot of Beastkin? (I assume they fall in love with another race but bc of the "Xyz and Beastkin can't marry" or whatever they don't have children and the population dwindleds or whatever the hell) (it implied right??!?)
Could the third party be that sleepy guy? Nagisa?