If you ask me (not that any did), until you have faced that kind of situation, you really can't speak about under-age sex.
I agree, Fox. I have a niece facing the same situation, but she is pregnant, the father is mentally ill, her parents are divorced, and she is going to have the baby (her decision). She is the sweetest, gentlest niece I have. It's heart breaking to see her suffer through this. She was under 18 and not ready for the responsibilities of sex.
I don't know if I said this here in this topic or in another topic, but I repeat. There is a big difference in maturity between teenagers in high school and those out of high school and in college. (18 being that magic age for most youngsters)
Either we teach them real responsibility at an earlier age, or we discourage intercourse until they are mature enough to handle the responsibility.
But, of course, we inform them about birth control etc, in case they don't listen to our advice (and we know that many teenagers won't).