Stop portraying women as sexual objects on nearly every commercial, movie, sitcom, song, book etc... Stop making sex seem like it's the automatic next step in a male/female relationship no matter what the age. Stop portraying sex as this satisfying interaction- when in reality it is only satisfying if there is intimacy, and total trust, and isn't tainted by miles of sexual baggage from previous relationships.
Basically stop lying to kids about sex-and stop telling them that it's OK and expected for them to have sex... stop telling them with and without words that abstinence isn't a realistic goal.
Sex is mainly for procreation, and teens rarely hear about that aspect (thought they are bombarded with adds for birth control)- they see/hear about it terms of pleasure from a perspective of boredom and desperation.