As soon as you start trying to control how kids live, they will start fighting you. In my opinion kids are getting sexually active younger and younger because they keep getting told more and more how bad it is.
I'd like to mention that the the first post, which is talking about "controlling kids", is exactly the attitude that causes kids to do these things to start with. Parents should never try to control their kids, governments should never get involved with controlling kids, and religions and schools should never get involved with controlling kids (Sex Ed is fine as long as it isn't preaching abstinence).
Every parent is doing it wrong. They are doing what their parents did to them, which is wrong. They're piling empty second-hand knowledge onto their kids and trying to steer them according to their life experiences, instead of letting the kids make their own life experiences.
Humans learn from experience. Second-hand knowledge, rules, values, are nothing but a burden to be rebelled against. What's the psychology here? I would suggest a genuine curiosity. "Is [suppressed thing] really as bad as they say it is?" So they try it. "That wasn't bad at all! In fact, I kind of liked it. Why would they lie to me? Have they even tried it themselves?" And so the trust is already broken beyond repair, whoever has told them this thing is bad or wrong is suddenly perceived as ignorant, and a distaste for authority has begun.
You raise kids by letting them do as they please, and only providing guidance and advice where necessary, and wisdom when asked for. Making mistakes is not a problem, that's where experience comes from. If left to learn things themselves, they will make mistakes, learn from them, and never make that mistake again.
That way, they'll ultimately grow naturally and healthily into an intelligent and perceptive human being, instead of becoming an obedient mindless robot or a delinquent aggressive rebel.
Now, sex is very much associated with being "naughty" or taboo.
The fact that parents, schools, religions, the media, etc. keep going on about how wrong it is to have sex at a young age is basically only going to make it sound more appealing. Especially at that young age when the kids are starting to feel the need to rebel against something... taboos regarding sex have basically made it into a very obvious choice. Other obvious choices include alcohol, smoking, and various drugs, all of which I have observed becoming prevalent at younger and younger ages.
tl;dr "controlling kids" is a fucking stupid idea and only achieves the opposite of what is intended