I fantasise about non-human partners, because they don't have to have a gender. I've heard a few people get hot about tentacles... meh... I think I can see that. It depends on my mood but those alien pics ^^^ are quite hot tbh. It's not like you're gonna marry them is it?
There are plenty of steamy romance novels about marrying an alien, actually. I'm a fan of the "mars needs women" trope myself - that's the one where the aliens have a shortage of females for some reason, so there are handsome young alien men wanting to marry Earth women and have children with them.