What would I do? Sure, why not as long as I'm here doling out parental advice
I'm not a parent but I have been a teenager (more recently than many here, I'd wager). I think it comes down to overall philosophy re: parents and children. I am friends with a couple that clearly subscribes to a non-interventionist approach with their four and two-year-olds. My observation is that this method is EASIER FOR THE PARENTS. They do not have to bother settling the children down when they're acting up in public, do not have to take them outside, and simply ignore the disapproving stares of the passersby wondering why the parents don't control their little monsters.
This, punctuated by idle threats of countdowns to "time out."
Then, we have the "parent as authority figure" approach in which the children understand that it is not they, but the parents who are the boss. If this concept has not been instilled in the child by the teenage years, then yes, by then it may be too late.
Teenagers who have sex are also more likely to drink, smoke, and do illegal drugs-- basically, sex is only one symptom of an undisciplined child. Don't misunderstand, by undisciplined I don't mean by the parent, I mean a child who is unable to exercise discipline. This lack of discipline will carry over into other facets of his/her life.
"I know I shouldn't be having sex... but it makes me feel good." Extend that reasoning to other teenage issues. Extend it to their adult lives.
So short answer, I would HOPE that my child would be wise enough to not start having sex at such a young age. But if he did I would make clear the need to stop such behavior and the benefits of doing so, and he would have 14 years of discipline and education backing me up on that point.