A romance, sexual or otherwise, between two different species.
Sometimes they are Humanoid Aliens, or they can be a Friendly Neighborhood Vampire and a human, two Funny Animals, or a Beast and Beauty couple.
If society does not approve of such romance, then it's also Maligned Mixed Marriage.
Interspecies Friendship is the platonic version of this trope.
The man is dark, twisted, and eager to take out his nasty, sadistic, cruel fantasies on the helpless bound beauties on display before him.
Chances of it actually becoming an Official Couple (and evolving into a mixed marriage) are increased if the nonhuman has a human form.
Alien invaders seemingly love to tempt humans with this as part of their Masquerade, although you'd think they'd learn better.
See also: Extreme Omnisexual, Bestiality Is Depraved, Mars Needs Women, Divine Date, Magical Girlfriend, Deus Sex Machina, Did You Just Romance Cthulhu?
THEY WANT TITS AND ASS, AND WILL TAKE THEM BY NO MATTER HOW! But every good slave worth her holes needs to be sold.
Sandy sat up in her chair as she looked out towards the city. The white slaver offers them to an ugly, filthy, dirty man.