From puberty on, sexual activity is one of the strongest motivational factors in human behaviour, at times outweighing the urge to eat or drink. "Just say no!" is as absurd as when it is used concerning drugs. Throw drugs and alcohol into the mix and all rational goes out the window. The urge tends to fade once we get older (or about two months after most people get married). The problem of unwanted pregnancies is far to complex an emotional and psychological problem to be solved by buzz phrases like "keep your pecker in your pants".
It's called parenting.
add:
7 kids and no unwanted, surprise teenage pregnancy's. Probably just luck though, right?