well it doesn't take a rocket scientist to figure out that plenty of people who pledge abstinence until marriage end up having sex before marriage anyway. When did they pledge it? High school? Then did they go to college? People change over time. You can be immature and not ready for sexual relationships at 16 and thus say you won't have sex until marriage, but then when you are 20 you might feel a lot more mature and able to handle such relationships, at which point you reėvaluate your choice. It happens all the time.

Anyway, I think the point of the study is to say that when we say "abstinence is the best way to avoid STDs" (which is clear) we ignore the fact that once you do have sex, which many people will do regardless, the best way (besides knowing the sexual history of people you sleep with, monogamy, etc) is to use a condom. By only teaching abstinence and saying that promoting condom use undermines abstinence education, you end up failing to adequately combat STDs. That conclusion seems to support the statistics used here.