Even in a vacuum, "human nature" arguments for individuals make very little sense against things humans already do or want to do. Here we have humans which are pretty natural doing that very same thing, so in order for them to not obey "human nature" they'd need to have been influenced by space aliens or something.

Obviously Nazis effectively already believe that, but it's odd how "well meaning" libs often fall into that same trap.

Sea Lions and Giraffes and Bonobos have plenty of gay sex, Rams too, why is the line drawn at humans?

probably because we're supposed to be "better, intelligent and more civilized than animals", whatever that means 🙄🙄🙄

Might be because the reactionary version of human nature is the official kind of human nature of the west during early to mid capitalism, which is then universalised. Not only are they not considering your point, they're also ignoring that fact that for most of human history, it's been 'human nature' to be quite gay. Or at least to be very close roommates.