I was having a chat about this very topic with some friends recently - we all worry, whether parents of boys or girls about this.
We all said that the first few erotic things that we were exposed to around the age of 12-14 or so did really "set the scene" for our adult sex life. Luckily, for most of us it was "The Joy of Sex", Jilly Cooper books, Dirty Dancing, Wish You Were Here (the film, not the travel show..) seeing Jason Donovan with no top on and at the most raunchy end, Eurotrash and The Word.
So maybe the answer is to try to (in some way) allow children in their early teens, before they have had exposure to harcore porn, to see "lighter" erotic images and stories so that their initial sexual experiences aren't based on redtube and youporn.
Because surely if you're seeing anal sex at 12, what on earth are you going to be looking at by 18?
How you do this though, without being totally creepy, I have no idea.