1. Be open about it. Don't be afraid or ashamed to talk about it, because your child might then feel that it is something to ashamed about. That doesn't mean it will inhibit them from doing it once they decide to, but it does mean that they'll be less inclined to talk to you about it.
2. Be honest. If you would prefer that they wait for marriage, TELL THEM. But also let them know that you'll love them no matter what they choose, and you'll be there for them.
3. Express your deep concern that no matter what they use birth control. Whether they tell you or not, whether it's in your belief system or not - because if birth control isn't, I doubt that a child before marriage is either. Of utmost importance is THEIR safety, not your peace of mind. There are so many diseases out there, so many risks for teenage pregnancy, if nothing else they need that protection.
4. Express some kind of belief that they will make the decision that is right for them. Don't try to order them to do anything - that only causes rebellious feelings. Let them know that in this, it's their judgment that matters.
5. ASK them to come to you when it's time so that you can provide the birth control. If you are honest and open and they don't feel like they'll need to be afraid of your judgments or that you might not love them anymore, then they will come.
Most importantly, remember that this is about them, not about you. Because no matter what you do, they are going to make their own choice on this. You cannot be there every second of every day, making sure that they are following your wishes on sex. They see it on TV, in movies, hear about it on the radio, talk about it at school with their friends. In the end, it's their judgment that you're going to have to rely on. So you can either be a part of their life in this, or not. If they feel that you'll judge them, that you won't accept them, that you'll deny them if you find out that they've been having sex, then they won't come to you. They won't talk to you about it. And it's a much higher chance that they'll be irresponsible about it.
And whatever you do, don't panic. You act like it's the end of the world and I can almost guarantee that your kid is going to rebel. Or that they're going to be so afraid of you that when the times comes, they aren't going to be able to talk to you about it at all.
You may not believe it's right... but I hate to tell you: THAT DOES NOT MATTER. Your kid is going to do what he or she wants anyway.
This is the best advice I can give to parents who are trying to deal with their teenager's sexuality. Be open, be honest. Tell them that you weren't perfect either. Tell them that you waited till marriage. Tell them about your co-worker's daughter who's pregnant at 15 because she didn't use birth control. Make sure that they know about AIDS and STD's. Give them the knowledge and the support that they are going to need, because this is about them and what choice they're going to make. All you can choose is to be there for them, or not.