All of the underlying games could be run either in DnD 3.5 with use of The Guide (to UCN), or in the Futaformations system found here: http://futaformation.pbwiki.com/
A Brigand and a Silver Dragon: For Two People
(For purposes of this description, the brigand will be described as male, and the dragon as female, this need not be the case)
A suave ne'er do well has been 'patrolling' a section of highway, robbing those that seem poorly protected, and generally doing quite well for a robber functioning solo... Until she showed up during one of his raids. She was quite attractive, and almost naked, with skin of silver scales. She seemed to think... Poorly of his actions, his selfishness, his violence towards others, and she seemed rather intent on punishing, or perhaps killing him. While he was... Proficient at what he did, it was clear she was just a human form of a Silver Dragon, and not trying to hide it, either, and so he was certain that in a physical confrontation he would likely meet a rather swift and painful death... And so he tried to talk her out of it, seduce her, anything. It goes well for him, perhaps a bit too well, as she's quite delighted with his performance, and decides that for her pleasure, and for the good of the populace, she'll 'take care' of him. He finds himself taken to her cave, treated well, but definately the... Inferior of the pair in everything except bed.
I expect the story to follow her slowly emerging sexuality, and his attempts to escape, or take advantage of the situation, etc. Again, roles could be easily reversed with a female bandit and a male dragon.
Trouble in Court: For... Well a decent number of people
A group of adventurers have been invited to the King's court, as strange things are afoot in the castle. There have been reports of people dissappearing, being in multiple places at once, and the usually... Steamy side to the politics is quite a bit more severe than usual. The adventurers are being treated as a few lesser nobles and their staff so that they may carefully investigate the situation from within...
Probably best for a DnD setup, a series of mysteries and secrets with pleanty of time for side romps with the serving maids... Or the Dukes, or Duchesses. This game would be pretty heavily shaped by the group I get, but I have a rather amusing castle and cast, and such planned out in my mind.
A Demon's Mates: For two to four people, I think. Potentially extreme-ish, depending on the wants of the players...
A demon is collecting partners to help create a nest of its own earthly spawn. It moves into towns, finding men and women who are lacking a connection to others, or simply seem unworried about, and seduces or rapes them, and spirits them away to help its breeding project.
Details are sketchy, I'd think Futaformations, possibly even using some of the enhancement/humiliations stuff if people want, a rather flexible game setting, really...