As a parent, all you can is to lay out the facts for your child, and hope they take notice.
I told all my girls the same - regardless of the pleasure aspects, intercourse is a biological function, and its purpose is conception. With that in mind, the odds are serious stacked IN FAVOUR of conception, so here's a piece of perspective. If every sperm in the average ejaculation fertalised an egg, the result would double the population of North America - not the U.S., the continent of North America, including Canada, the second biggest country on earth.
Put it like that, it gives a teenager some idea of the odds of conception without protection, so hopefully they understand that contraception is the adult responsibility that goes with the adult act of intercourse.
Tell your teenager you will answer all questions honestly and fully if they wish to ask, but to remember that pregnancy and STD's don't 'happen to someone else' - they happen to anyone who is out there, so they MUST PROTECT THEMSELVES.
Now comes the hard bit - you have to let them go out and do what they will do, and hope they have listened to you, and that is really all they can do. Scary isn't it?
But if you have raised your children to be responsible and aware, they will know the risks, and deal with them.
Good luck.