First, it's very important to have a good relationship with your teens, since it increases the chances that they'll identify with you and your family's values. If your relation with them is not what you'd prefer, know that you can improve the relationship and it's definitely worth it. As you raise your children and teenagers, steer them toward interesting, clean activities that are age-appropriate, and encourage them to set goals and do well academically. If they have a bright future to look forward to and healthy self-esteem, they'll be less likely to use sex to solve their problems. Monitor the friends that your teenager hangs out with, how your teens spend their free time, and what kind of movies, videos and television shows they watch, without being heavy handed.
Next, get clear in your mind how you'd feel if your teenage daughter would become pregnant while still in high school (and possibly undergo an abortion), or if your son would father a child and stop his education to get a job to support his girlfriend and baby. Look up statistics of date violence and STDs, and read stories about teenage girls who opted for an abortion and then suffered guilt and secondary infertility. You can start by sharing what you think of whatever television show or movie you're viewing with your teen, and point out the underlying messages that conflict with reality and your values. If you believe that people should wait until they're married to have a physically intimate relationship, you can tell that to your teenagers; you can also explain to them the difference between love (closeness, intimacy and commitment), infatuation, and when a boyfriend/girlfriend uses them for his/her own enjoyment (attraction and passion).
Realize that the society we live in bombards teenagers and adults with suggestive images and messages of instant gratification and irresponsibility. No one said being a parent is easy…