According to this Reuters report:
"The largest and most in-depth report on US sexual behavior, sexual attraction, and sexual identity shows a rising number of young people are electing to remain abstinent. In 2006-2008, 29 percent of females and 27 percent of males ages 15-24 reported not having any sexual contact, compared with 22 percent in 2002, CDC said Thursday."
The study also reports a sharp rise in same-sex sexual activity among women: 13 percent of women reported every having had same-sex sexual contact compared to 5 percent of men.
The report, “Sexual Behavior, Sexual Attraction, and Sexual Identity in the United States: Data from the 2006-2008 National Survey of Family Growth,” was published in National Health Statistics Reports (2011:36). It can be found here.