Some argue that vaccinating girls for HPV would give them a false sense of security, and will thus lead to young teens' having more sex. I'm pretty skeptical about that.

Those girls who think about the risks of sex (and who knew in the first instance that sex can cause cervical cancer) will still realize that sex can spread AIDS — which kills roughly 2300-2500 American women per year, which causes over 5000 more per year to fall ill, and which strikes me as a much more publicly focused on risk than is HPV. They'll also recognize that sex can cause pregnancy; and those who blithely have sex without paying much attention to these remaining risks likely would have blithely ignored the risk of cervical cancer, too. "I wasn't going to have sex, because I was afraid of getting cervical cancer or AIDS, but now that I know that I'll only be at risk of AIDS, I'll go ahead and do it" strikes me as an implausible reaction.

In principle, I'm open to arguments that legal requirements that seem to increase safety will lead to riskier behavior that will eliminate much of the safety benefit. That's certainly a danger that sound policy analysis should take into account. But in this particular situation, it just doesn't seem very plausible.