This is just a bit of a starter right now, I'll add more, but I'm going to have more information in my ONs and OFFs page here in the beginning. Right now, I'm just going to post some pairings with a little plot clarification. Green marks ones I crave.
Angel/Demon: My character is the Angel, yours is the Demon and you've outwitted my character, captured them somehow with the intent of
using them as a trophy sex slave, different from an incubus or succubus.
Anthro/Anthro: This can be pretty much anything. We can be any kind of anthro (same species or different) in any kind of setting you'd like. Suggest something, I've got a few vague ideas.
Human/Alien: Your character is an alien in this science fiction setting. It could be that they find themselves on the same crew of a small vessel and they just get along way better with one another than with the other crew-members. If you have a different concept for this one (perhaps a science fiction version of the Angel/Demon plot), go ahead and make the suggestion
Alien/Alien: Same as the Human/Alien, but my character is an alien as well.
Superhero/Supervillainess: This one could work a few different ways. It could be a bit of give and take in the super aspect of their lives until, unbeknownst to each other, they meet in their secret identities and hook up. They start dating, all the while continuing to fight one another, until they start falling for one another. Once that happens, that's when they find out who the other is. The alternate way I'd like to do this is for your supervillainess captures my hero and, simply, has her way with him going on to train him to be subservient to her and rule the world.
Human/Drow: Again, two different ways to do this. Maybe your character is one of the good drow and meets my character, perhaps a little nervous of meeting an actual drow, but they start to get to know one another and romance ensues. The other way is that my character is just unlucky enough to be captured by your character and owned as a servant/sex slave. Simple as that, he becomes a status symbol.
Student/Teacher: Senior year of high-school or college, maybe it's more than just sex, maybe he's doing it for a grade, it's up to you.
Again, there will be more to come later, this is just a seed, planting it here so I can continue to work on it in the future, adding more pairings as well as more specific plots.