According to the latest research, programs that teach abstinence only do not decrease the HIV infection rate, or the rate of unprotected sex, or even change the initiation of sexual activity.
Now, I sort of thought that this was something to file under the "duh" category, myself. But apparently there are those who believe that "just say no" works. Apparently, these people think that by telling a teenager to not do something, that teenager is going to not do it. What sharp people these are. These people also operate under a presumption that the whole world is just like their little corner of it, and that nobody has it any worse off than them, except for the poor choices those others have made, such as to be born in rural Thailand or Uganda. You see, they should know to say "no" to unprotected sex, because we teach them to say no.
I'm all for teaching your kids to wait. But you have to plan for contingencies. You teach children to swim so that they don't get hurt playing near water. You teach children about venomous snakes so they know which critters to avoid. You don't just say "stay away from snakes." An ounce of prevention, and all that.
I'm going to go file this back in the "duh" category.