At the same time, Congress and the Bush administration have directed hundreds of millions of dollars toward abstinence-only education in the public middle schools and high schools  classes that have been roundly criticized for blurring the line between science and religion. A 2004 report issued by Representative Henry Waxman, a California Democrat, found that 11 of 13 abstinence curriculums that his government-reform committee examined were rife with scientific errors and false and misleading information about the risks of sexual activity. Many states are now rejecting federal financing for such classes, on evidence that they fail to limit sexual behavior or reduce teen pregnancy.

In a follow-up study to a 1995 national survey of close to 12,000 students in grades 7 through 12, two sociologists, Peter Bearman at Columbia University and Hannah Brückner at Yale, found that while those who took virginity pledges preserved their technical virginity about 18 months longer than teenagers who didn’t pledge, they were six times more likely to engage in oral sex than virgins who hadn’t taken a pledge. They were also much less likely to use condoms during their first sexual experience or to be tested for sexually transmitted diseases. Disease rates between those who pledged and those who didn’t were actually similar. The authors, who published their findings in 2005, concluded that the emphasis on premarital abstinence was insufficient to fend off disease and “collides with the realities of adolescents’ and young adults’ lives.”

Many college students today, however, grew up with abstinence classes and clubs in their communities, and so the movement has raised a generation of activists. Among prominent abstinence activists is Wendy Shalit, who wrote “Girls Gone Mild: Young Women Reclaim Self-Respect and Find It’s Not Bad to Be Good,” which came out last year. She says that talk of disease rates and the amount of sexual activity on campuses is beside the point. A sex-saturated popular culture creates certain expectations, she argues. “The key thing to remember,” Shalit wrote me recently in an e-mail message, “is that many young people involved in sexual activity feel pressured into it.” Many are uncomfortable with “the hookup scene,” she continued, and “college abstinence programs are growing out of this awareness that disconnected sex is not as pleasurable as the media (and sometimes college administrators) have led us to believe.” The awareness is especially acute in the highly politicized environment of the elite schools, where, according to Shalit, “there is just one lifestyle that doesn’t get recognition”  premarital abstinence.

The Ivy League’s abstinence clubs began emerging several years ago about the same time as student sex blogs, sex columns and, at Harvard and Yale, student sex magazines. Those involved, however, say that the most important catalyst was university-sponsored safe-sex education, which they saw as institutional encouragement of promiscuity. The founders of the Princeton club, the first to form in the Ivy League in 2005, wanted to offer an opposing view. Many were Catholic, but seeking credibility within the university at large, they decided not to present themselves as a religious organization and always to “shy away from arguments with religious premises,” says Kevin Joyce, a former president of the club. “Here at a university, we have to provide the intellectual basis” for abstinence, he told me. “Every position we take as a group can be confirmed by rational thought.”

Image Janie Fredell, an advocate of premarital abstinence, says that "virginity is extremely alluring." Credit... Katherine Wolkoff for The New York Times

Making a rational case against premarital sex was easier before reliable contraception. But to shore things up, the club has turned to Catholic thinkers like Elizabeth Anscombe, the philosopher and student of Ludwig Wittgenstein. Anscombe’s arguments against premarital sex are as impressive as they are difficult to summarize, and the students so admired her logic, they named their society after her. Robert George, a professor of jurisprudence at Princeton, is one of the Anscombe Society’s informal faculty advisers. Himself a Catholic thinker, George says that society members employ “philosophical-ethical arguments” to support their belief that promiscuity “deeply compromises human dignity,” and psychological and sociological rationale to justify the claim that casual sex leads to “personal unhappiness and social harm.” The students are some of Princeton’s most gifted, George says, and “even people who don’t accept their conclusions recognize that the arguments being advanced by the Anscombe students are serious and cannot be easily dismissed.”

The Anscombe Society at Princeton went on to embrace positions not just against premarital sex but also against homosexual sex and marriage. Founders have tried to spread its method to other schools, and students at the Massachusetts Institute of Technology were the first to follow with another Anscombe Society. Bill Jacobs, the president, says it’s a loosely organized group. “People tend to be pretty busy with homework,” he says.