If you read the article, they are comparing teenagers who took abstinence pledges TO OTHER ABSTINENT TEENAGERS, not to teenagers in general. Sure enough, they managed to prove that pledge kids are misbehaving. This gets translated into the headline "Abstinence causes risky acts," though both groups in the comparison are supposedly abstaining. Only in liberal la-la land can you compare kids who promise to be good with kids who actually are good, find the promisers coming up short, and say "being good is really being bad".
Probably all this proves is that many of the abstinence-pledge kids are operating from peer- or parental pressure, not from actual conviction.
For the libertine "all teenagers have sex, you can't stop it" crowd, the fine print in the article points out that 88 per cent of abstinence pledgers wait until marriage, versus 1 per cent of the population as a whole.
(fixed)