Teens have free will. There is nothing you can do to keep them from having sex if they want to. Unless you plan to lock them up and keep them away from the rest of the world. A parent's best hope is to instill values early on and make sure your kids feel comfortable enough to talk to you about anything.
Odds are most teens will have some type of sex. It is that period in their life where hormones are raging and many them also believe they are in love with the person they will some day marry. You also have a lot more media outlets, young celebrities, music. movies/television shows which promote sexuality more than ever.
Personally speaking I think you have better odds of preventing teen pregnancy or getting STDs by having conversations regarding safe sex. I grew up during the 70s BEFORE there was MTV or Hip/hop music and I had sex as a teen as well as most of the teens I knew in high school. (My mother was super strict!)