The findings are especially striking — and puzzling — since the previous round of the survey, in 2002, found no pattern of educational differences in women’s sexual behavior. Most of the change came from higher levels of same-sex behavior reported by the women without diplomas.

“I always thought the LUG phenomenon was overblown, in the context of it being erotically titillating for young men,” said Barbara Risman, an officer of the Council on Contemporary Families and a University of Illinois at Chicago sociology professor. She added that the new findings may reflect class dynamics, with high school dropouts living in surroundings with few desirable and available male partners.

Amber Hollibaugh, interim executive director of Queers for Economic Justice, a New York-based advocacy group, said the results of the federal survey underscored how poor, minority and working-class lesbians had been overshadowed by the mainstream cultural image of lesbians as white professionals.

“Working with a gay-rights group is now something you’d put on your résumé,” said Ms. Hollibaugh, who did not attend college. “Lesbians who aren’t college-educated professionals are pretty much invisible.”

Dan Savage, a gay sex columnist in Seattle, said the LUG phenomenon may be overrepresented in the national imagination because so many students sought attention for their sexual exploration: “A lot of them are out to prove something and want their effort to smash the patriarchy to be very visible,” he said.

Lisa Diamond, a professor of psychology and gender studies at the University of Utah, said that with gay relationships so much more common throughout society, college campuses may have lost their status as the “privileged site” for women’s exposure to different kinds of sexuality. “Maybe our stereotypes are just behind the times,” Ms. Diamond said, adding that while lesbian and gay couples raising children were still assumed to be sophisticated white professionals, as in the movie, “The Kids Are All Right,” the latest parenting data showed that “holy-moly, it’s less likely to be upper-middle-class same-sex couples than ethnic minorities and working-class couples.”

Most headlines about the report, released earlier this month, focused on a finding that young people were waiting longer to have sex. Almost 29 percent of the females and 27 percent of the males, age 15 to 24, had had no sexual contact, an increase from 22 percent for both sexes in the 2002 survey.