The "they're a different species from humans, they might not even be able to successfully mate with them" thing really doesn't apply here. These are humanoid cryptid species that are so close to humans that interbreeding is 99.9% on the table unless the author decides to rule it out themselves. This is all about genetic purity. I used the wrong phrase, and that was my mistake. In fact, Vivi's situation also has to do with genetic purity, because her family sees her as "broken" because of her differences from "normal" vampires, probably brought on by genetic mutation. I wouldn't be surprised if "genetic purity as an idea is toxic" ends up being the central theme of this entire series.