my DS has learned the F word, and about sex at the school playground (very "nice" school, little village primary, you get the picture).
He told me that it is about a man peeing in a woman. and that to do "it" you had to pin a girl against a wall and rub yourself against her (OMG!). he was quite horrified. So was I
.
So we had "the" chat, age appropriate, as I'd rather he gets the facts from me, in a non-sensationalist manner, than believing what all the boys say in the playground. It also gave me a chance to explain about appropriate behaviour, and that pretend-humping (what they were all doing!) is inappropriate and could get them into trouble. I also had a word with the teacher, and she says it is normal in 8 year olds, and they have "chats" about appropriate behaviour at school.
I know lots of parents who are in denial about what their children know, especially parents of girls. Especially as we live in the countryside, and you see sheep, horses, cattle, dogs doing "it" everywhere.