Um.
Unless I'm missing something, the only way abstinence-only education can reduce teenage pregnancy is by reducing teenage intercourse. Or encouraging homosexual intercourse instead of heterosexual. Or encouraging sexual behaviors other than intercourse.
This isn't the first study to find flaws in abstinence-only education. I'm more a fan of the belt-and-suspenders approach -- encourage abstinence in the strongest possible terms, of course, but also teach contraception. And don't buy the argument that teaching teenagers about contraception encourages them to have sex -- they need no encouragement.
By way of analogy, imagine an "abstinence-only" approach to driver's ed: "Don't drive like an idiot," end of lesson. As opposed to "don't drive stupid, and always wear a seat belt." Do seat belts encourage stupid driving?
Your analogy is somewhat flawed because wearing a seatbelt is wise advice for both safe and unsafe driving. A closer analogy is if I, as mom, tell my kids “Don’t wander around at night, but if you do go, wear a jacket.” What’s the kid going to hear? As long as I wear a jacket, I can go. They believe I have given permission to wander around so long as they take certain precautions. And it is this wholesale acceptance by the so-called adults of the world that kids will have sex regardless which is destroying these kids. Pregnancies and sexual diseases, while tragic, are not the worse result of this acceptance. Rather it is the destruction of these kids’ ability to create meaningful, lifelong commitments to each other that will cripple this society and the generations to come.