Update - another idea!
This was on someone else's list and I sent them a private message, but it seems they haven't gone online in a while. So I thought I'd throw the idea out for general play:
Concept: There is a wandering half fiend who has just the smallest bit of good left in his/her soul, the remnants of the non-fiend side (human, elf, whatever). A female paladin (or paladin-like character) encounters the fiend and naturally attacks, choosing the good old "Smite on Sight!" that never failed a paladin yet. The two battle, possibly several times, before the paladin finally realises the half-fiend is pulling his/her punches and is possibly redeemable. She dedicates herself to helping curb the demonic side and bringing the non-fiend side out and into control. However the fiend still has dark urges that will consume the body if not released through some outlet, so she uses herself - through rough, nonlethal combats that leave the two naked, exhausted and mildly scratched up, and sometimes through allowing it to have its way with her. However, to help tame the fiend and bring out the other side, she also does the same to it from time to time.
I don't mind playing either, though I envision being the paladin-like character. If the other would rather be the paladin-like character though, go for it. For the half-fiend, the only restrictions I place are "look mostly humanoid" (growths of scaled armour is fine, spines growing in places, burning eyes, wings, a tail, horns, fangs, claws, they're all fine.), and be fairly pretty - for either sex. If a male, they should be a pretty-boy. Eventually they would probably form a relationship.
System: D&D or freeform. I think D&D would probably be best for this (besides, I like spending time making character sheets, it gives me something to do). Mid levels probably, with the paladin-like character (Paladin, Crusader, Soulborn or Warblade would be the best ones) being around 8-10th level and the half-fiend being 2 or 3 levels lower (but the half-fiend template bumps them up to being a bit stronger). Combat must be described in a dynamic fashion, and should also be made a little sexy, with clothes being damaged, lots of close-up grappling, with fingernails/claws running over each other, a lot of kicks if short skirts/tunics are involved, characters getting turned on as they fight, accidentally striking (with the flat of the blade, or unarmed open palm attacks) erogenous zones... you get the idea.
Naughtiness: Erotic fighting, a little bondage, not-completely-consensual sex (the paladin letting the half fiend ravage her to let out its lust and wrath, not out of a desire for it... to start with), a bit of pain - spanking/whipping kind of stuff, nothing severe.
Duration: Probably not that long a game, just a few scenarios/events, and then all is wrapped up and archived, a happy completed story.