Here's the most intriguing part of the results, for me:
The analysis also found that in communities where at least 20 percent of adolescents pledged to remain virgins, the STD rates for everyone combined was 8.9 percent. In communities with fewer than 7 percent pledgers, the STD rate was 5.5 percent.So it's precisely in communities where there's a lot of vocal support for teen abstinence that, apparently, you have the greatest rates of unprotected sex. That doesn't surprise me at all. The study's author thinks this finding is related to socially-enforced hypocrisy - the old "swept away" phenomenon, where using birth control means that you intended to have sex, which means that you're a whore. Presumably, communities with a lot of vocal support for teen abstinence also tend to condemn girls who have sex, thus increasing girls' determination to only have sex if they seem to have been "swept away."
Another way to understand this result is by looking at the importance of peer norms for sexual behavior. We know from repeated studies that people, including teens, are more likely to use condoms if they think that their friends use them. It's the upside of peer pressure. If all your friends think that having condomless sex is crazy, you're going to feel more comfortable insisting that your boyfriend wears one. At a more basic level, you're simply more likely to think about using condoms, and talk about them with your partner, if your social circle considers them to be a standard feature of sex.
One of the most effective HIV prevention interventions took advantage of people's tendency to follow peer pressure. Instead of sending public health educators into the gay community, they went to gay bars and had the bartenders pick out the most popular and well-liked men. Those men were then trained to promote condom use via casual, one-on-one, personal conversations. When the popular guys became associated with promoting safer sex, everyone wanted to do it, and higher rates of condom use were seen throughout the community - well beyond the folks who actually received the intervention.
Abstinence pledges are the same thing in reverse. If the popular kids are publicly swearing off sex, there's an absence of social norms for condom use. Even kids who didn't take the pledge will hear fewer positive messages from their peers about condoms, and will have less of a sense that condoms are widely accepted in their peer group. It's hard for teenagers to do things that aren't widely accepted in their peer group. So high rates of abstinence pledges in a community make it harder for all the kids, not just the pledgers, to plan for and insist on condom use.