"The biggest surprise to me was the answers to questions about female-female sex," said lead author William Mosher, 54, "The previous surveys had not shown as much."

The study, conducted in 2002 by the National Center for Health Statistics in Hyattsville, Md., also found that slightly more than half of American teens aged 15 to 19 have engaged in oral sex.

* 49 percent of males have had vaginal intercourse, while 55 percent have had oral sex with a woman. 53 percent of women have had vaginal intercourse, while 54 percent had oral sex with a man.