I was reminded of how i can totally be sold on any pairing provided the fic selling the pairing is written well enough (which likely comes from the fact that my primary fandom is Whedonverse where everyone has sexual tension with everyone) so the UST becomes canon desire in my head and then it's a quick step to "Well why wouldn't they act on it?"
Also, (1) i was talking with immortalavalamp recently about how in a lot of Trek fic you want certain characters together but because said characters are so honor-driven or ships' captains or whatever it they really wouldn't get into a sexual relationship with said other person, and (2) i'm writing Giles/Cordelia in which Buffy is dating Riley but still has a crush on Giles and i'm debating about how totally out of character a Giles/Cordelia/Buffy threesome would be.
P.S. I totally just got sold on Charlotte/Kelly from Lost in Translation. Damn.
Paper, yes, really.