Ah, the naivety of youth.
So if you meet someone in six months and decide to have sex, you really think your parents would be able to prevent it?
I bet you'd be on here in a flash, complaining about how they wouldn't even let you be alone with him for five minutes and treated you like a little kid. It's impossible for a parent to prevent an older teen from having sex, not if they are going to allow them any sort of freedom and responsibility - which they need in order to mature. All they can do is inform. And, to be honest, I'd be more inclined to buy my child a condom than to shriek and scream about how they were too immature to have sex. In the second case, they're almost certain to want to prove me wrong. In the first, I'm making it very clear it's their decision -and if I've educated them decently, there's a reasonable chance they will decide not to.