This reminds me of the episode of The Newsroom last night, where Maggie asks "What's wrong with being a slut?".
If they're not having sex in the bedroom they're probably having it somewhere else or at the other persons house. There isn't anything wrong with sleepovers. Nowadays I sleep at my girlfriends on a regular basis and it helps build a relationship, if anything (granted this is a good one). I think being more open about sex and not framing it in such a negative light would be best for everybody. There's nothing wrong with it if done responsibly, and saying not to do it at all as a teenager is not treating it as the subject should be treated.