It's fairly well-established that having a close relationship with your kids (or, if you're a kid, with your parents) tends to keep those pesky teens from engaging in risky behaviors and delay the onset of sexual activity. The real question no one has really asked until now is what the definition of "close" actually is. It turns out that the definition of close might have a lot more to do with how you walk that line of showing them appropriate respect with still being a parents rather than being a strict parent. As my parents have been finding out since I've been writing here, being strict really doesn't keep your kid from having sex. According to a study by Stephanie Madsen at McDaniel College, parents fall into 4 camps: the permissive ones (whose kids run wild); the ones who set strict rules about their kids' dating habits and sexual contact (whose kids run wild); the ones who have unhappy relationships who try to set rules about their kids' treatment of the people they're dating (whose kids run wild); and those parents who are more focused on parenting and supervising their kids' transitions to adulthood without trying to relive or remake their own (whose kids are the most normal).
Basically, if you act like you trust you kid to make good choices and have open lines of communication where they don't feel like they have to lie, then they'll spend some of their energy trying to live up to that respect. If you give them a rule book, they'll just run around trying not to get caught breaking it. And, naturally, if you act like you don't care what they do, they won't either.
Look, if your teenager has responsible sex (and they will have some kind of sex) it isn't the end of the world — and it shouldn't be. It might not be in line with your moral judgment, you might think they are too young, whatever. I know my parents weren't (and probably still aren't, lo these many years later) happy about it. On the other hand, if your kid can't talk to you about it, or come to you for comfort and support because they've broken your rules and are so scared of punishment that they can't turn to you when they really need you, then you're neither keeping them from making a mistake nor helping them learn from it. And from someone who used to be a kid who couldn't in a million years talk to her parents, that just sucks.
What the Dating Rules You Set For Your Kids Say About You [Wall Street Journal]