It seems to me the sensible thing to do is both. Tell teens they should avoid having sex if only to avoid the very real threat of pregnancy and sexually transmitted disease, and at the same time, inform them about effective birth control methods because a good number of teenagers are going to have sex anyway despite anything we do or say. Further, don't demonize those who want to wait for marriage, and don't demonize those who find waiting for marriage antiquated.
Once they get to college, or at least out of the halls of our high schools, let them do whatever the hell they want, just as we do now.