Right around puberty, when I realized that "sex with yourself" and "that thing I do" were the same thing, my fantasies took their only plunge into vanilla. They did, however, stay sci-fi.
My fantasies have always been a continuous story, more or less, updated with about a half-hour of new content every night. I'm not one to go all anthology-style with anything that strikes me sexy; I write novels in my head, dammit, albeit incoherently lurid ones.
The first story was about a couple who can go between parallel universes. They spent a little time in places like The Universe Where Everybody Fucks Like It's a Handshake, The Universe With No Girls (until now!), The Universe Where People Have Sex Long-Distance Like Fish, and so on. Eventually, though, they settled on a universe that was just a big house, with perfect privacy, that they could change according to their whims. They called the place Alibi. Mispronounced Aleebee and I seriously have no idea why. I guess I thought it sounded exotic?
But during ages thirteen and fourteen or so, the stories from Alibi were gentle. The couple had lots of sex, but the variations were tame--oral sex, oh my, sex on the floor, how risqué, sex in the pool, ooh la la. Always fundamentally sweet and painless, never anal, never rough, just... the kind of thing I think you're supposed to fantasize about your whole life.
I got bored of it around fifteen.