My dd is just 7. We're quite picky about what she sees on TV but we were caught by surprise when Victoria Wood mentioned 'a couple having sex' on TV the other night. I glanced across at her & she did a 'oh how rude' face so I asked her what it meant. After a lot of pursuading (I probably made too big a deal of it but I really wanted to know what she knew!) She eventually told me it meant a man & lady kissing & cuddling 'for a really long time' (explanation courtesy of one of her friends, slightly younger but more advanced!).
I feel I need some help in the form of a good book to hand over if the subject comes up again!