This issue is related to incentive: recall that people react to incentive! It's on a teenage oral-sex craze in the United States by Tim Harford.
To sum up:
Both the adults and the teenagers are engaging in much more oral sex than in 1990. Why not?
Schools teach the risks of sex, particularly HIV/AIDS and some other sexually transmitted infections (STI).
Oral sex can be safer than penetrative sex, which dramatically reduces the risk of contracting HIV and reduces the effects of some other STI. An infection that might have made a girl infertile instead gives her a sore throat.
(a) The economic analysis
When the price of penetrative sex rises, rational teenagers seek substitutes. If teenagers really did think about the consequences of their actions, they would have less risky sex if the cost of risky sex went up; people who are still having sex have switched to using birth-control methods that will also protect them from STI.
The oral-sex epidemic is a rational response to a rise in the price of penetrative sex; use of the contraceptive pill is down by nearly a fifth, but use of condoms is up by more than a third. The percentage of teenage virgins has risen by more than 15 percent since the beginning of the 1990s.
(b) Policy implication
If teenagers are rational, abortion-notification or -consent laws would discourage risky sex among teens, relative to adults.
Some scholars found that wherever and whenever such laws have been passed, gonorrhoea rates in the teenage and adult populations start to diverge. When it becomes more troublesome to get an abortion, teenagers seem to cut back on unprotected sex.
Greg Mankiw's Blog: A Few Good Readings