Everything was going along well enough for our characters, Jason and Melissa were dating, had been for a long time, and had started to get intimate, while Vanessa (All names chosen randomly so they're not Girl 1 Girl 2 for this thread :p) has had a series of somewhat shorter, though more intense relationships, and was generally Melissa's best friend. The thing is, Melissa feels very self conscious about her, ah, performance with Jason, and knowing Vanessa is much more experienced and said to be quite... Good by her few previous companions, asks her for help. Vanessa offers to 'demonstrate' for her, show her the sorts of things she could do with Jason... With Jason, so she could get a feel for what Melissa should be doing. Melissa is quite tentative, but Vanessa cajoles her into it.
Unfortunately, Vanessa discovers that Jason feels pretty compatible to her.
Unfortunately, the demonstration session turned into something a little more intense than initially expected, almost Vanessa more flaunting her skills than sharing them.
Unfortunately, now Jason is stuck in a sticky situation where he loves one girl, but really 'enjoys' her friend.
And things can unfold from there.
Could be done with me and one other person, or we could get 3 people going at this, I have a bit of a preference to play Vanessa aside from whatever roles end up being required. Anyone interested (Details and names are, of course, negotiable :p)