Ok but the question is assuming they have a concept of species, presumably with a bright line, and then are using that to determine whether or not to have sex.

Ok but the question is assuming they have a concept of species, presumably with a bright line, and then are using that to determine whether or not to have sex. Which is circular (as well as absurd on so many levels). That said, I'm sure they had some concept of "differentness," which would lie on a spectrum, rather than the black and white way the above poster described it. Humans may, for instance, find people of different races to be less attractive in general. But exceptions can be made. I imagine if you are desperate enough, you widen the pool of acceptable candidates. I have no reason to think it was very different back then.