Reuters reports that the findings were based on a 2002 government survey of 2,271 females and males ages 15 to 19. It found that about 55 percent of the teens said they had engaged in oral sex, but the practice was far more common among those who also engaged in intercourse.

Teens even admitted they began the two at about the same time. Specifically, within six months of having intercourse for the first time, 82 percent had also engaged in oral sex.

"There is a widespread belief that teens engage in non-vaginal forms of sex, especially oral sex, as a way to be sexually active while still claiming that, technically, they are virgins," Laura Lindberg of the Guttmacher Institute in New York, who led the study, said in a statement. "However, our research shows that this supposed substitution of oral sex for vaginal sex is largely a myth. There is no good evidence that teens who have not had intercourse engage in oral sex with a series of partners."

In addition, about one in 10 of the teens said they had engaged in anal sex. These teens were also far more likely to have had intercourse. "Teens of white ethnicity and higher socioeconomic status were more likely than their peers to have ever had oral or anal sex," the researchers wrote.

"While oral and anal sex carry no risk of pregnancy, engaging in these behaviors can nevertheless put teens at risk of sexually transmitted infections," Lindberg said. "Counseling and education should take into account total STI risk by addressing the full range of behaviors that teens engage in, including oral and anal sex."

The Centers for Disease Control and Prevention reported in March 2008 that more than one in four U.S. teen girls is infected with at least one sexually transmitted disease.