Too many ignored it for too long, irresponsible and stupid behavior became the norm because of ignorance. Imagine if we were still ignoring the consequences other than the obvious of pregnancy like they did in the 60s with the same current of sexuality running through the young we have today.
You can, in fact, refuse to have your child learn of this in school. Even in the most "liberal" of areas you can simply deny them permission to teach it and either leave your child in ignorance of all consequence, or teach the consequences yourself. I am sure my children will benefit from knowing of this long before they get to this point in school. The dangers of it, the probability of being mocked for it, emotional danger as well as physical danger from disease.
When my children ask, I teach. Admittedly to the level of age understanding, but I teach directly and honestly so that they will know in the future that there is nothing, nothing at all, that they should fear to tell me.
The belief that it must be kept in the dark will only ensure that your kids will learn of it only from those who cannot understand the implications of their actions. Long before they ask you, they will have spoken of this with their friends and will feel sure that you simply "just don't understand". They will never truly trust you in matters where you keep them in the dark. The point that they learn this in school, they have already learned most of it from their friends, and in the most incorrect and, for christians, profane manner possible. Pretending that this is not so, so you can pretend that the scientific portion of it should never be taught, is silly pretense based on conveniently forgetting your own 11 year old conversations with your friends.