they’re both asperian- nux being half asperian/half human, Owen being full asperian- owen explained on that world they have soulmates and like in most omega verses one in the pair can get pregnant regardless of their sex. In this story however there seems to be a twist where like others have pointed out, the one putting it in got pregnant, what is happening is probably that the asperian can adapt their bodies for their mates and he’s developed a way to carry their offspring- perhaps similar to seahorses like some have said.