Maybe it's easier for teens to engage in sexual activity than it is for them to talk about it, whether with their parents or their partners. According to a recent poll of 15-17-year-olds, only one in ten say they have discussed with parents their plans to have sexual intercourse, and four in 10 sexually active teens have never talked about HIV/AIDS or other sexually transmitted diseases with their partners.
http://www.kff.org/content/2002/3240/