Here is an interesting idea to get teens to engage in less premarital sex.

In a study presented at a meeting of the American Public Health Association (APHA), researchers at the University of Kentucky followed 950 teenagers at 17 high schools in Kentucky and Ohio from 9th to 11th grades. They found evidence that teens who have intercourse tend to think their friends are too, even if they’re not. "You’re 2.5 times more likely to have sex by the 9th grade if you think your friends are having sex — whether or not they really are," says Katharine Atwood, assistant professor at the Kentucky School of Public Health. Plus, teens tended to overestimate how many of their friends were sexually active. Only 33 percent of kids in the study had had sex by the 9th grade, but 31 percent said that most or all of their friends had had sex. "If you can persuade them that fewer are having sex than they think," she says, "that can have a significant impact on their behavior."  (Italics added)

In other words…tell them that everyone is NOT doing it…just talking like they are.  I think it might work.

Link: Psychology Today: Teens and Sex.