There is a major difference between being mature and responsible enough to have sex, and just wanting to have sex and being ignorant of your own maturity or responsibility. All but a handful of the adolescents that I know (and a good portion of the adults, even), are ignorant of their responsibilities and the consequences of their actions, and simply do what "feels good" wether it's actually good for them or not.
Pardon my idealism for a moment, but think; if every human being on the face of the planet waited to have sex until after marriage, it would severely cut down on the spread of STD's, and abortions and unwanted pregnancies would be amazingly low.
Now, of course, that will never happen, because the average person just wants to do what feels good, and pretend that nothing bad could happen. And then when something bad does happen, they blame it on something else, and/or try to find the "easy" way out. A frighteningly small portion of humanity actually takes responsibility for their actions anymore.
I've spoken to a lot of adults on the subject (I'm 17, for the record), and I'm talking the mature, responsible, and succesful type of adult, and most of them regret having sex at such an early age. I'm more inclined to trust that they know what they're talking about, and follow their judgement, then to listen to a bunch of short-sighted, ignorant teenagers and young adults who can't even begin to know what's best for themselves, let alone tell other people what's best for them.
Moo.