Ten years of surveys with long names-the National Longitudinal Study of Adolescent Health, the National Survey of Family Growth, the College Women's Survey, and the College Social Life Survey-show that sex among young Americans is rampant, but it's not making many of them happy.

Two sociologists, Mark Regnerus (University of Texas) and Jeremy Uecker (University of North Carolina), crunched the numbers for their 2011 book Premarital Sex in America (Oxford University Press). They found that three-fourths of 18- to 23-year-old women are in dating relationships of some kind-and 94 percent of those are sexually involved.

Love and marriage no longer go together like a horse and carriage, but promiscuity and depression often do, especially among women. Women who have had multiple sex partners have poorer emotional health than women who report having had zero or one, and "women who report the greatest number of partners display the clearest symptoms of depression."

While it's simplistic to say more partners, more pain, a 2005 study found that "sexual behavior patterns came before depression, not after them. Girls with multiple sex partners were about 11 times more likely than virgins to report elevated depression symptoms." A majority of women who engaged in "hooking up"-casual sex-said they felt "disrespected" afterward. (One-fourth of men said they felt that way.)

Regnerus and Uecker found that men are typically in control of when dating begins, but women are in control of when sex begins-and it often begins earlier than they want. That's because "women are increasingly competing with each other for the affections of increasingly rare high-quality men who are willing to commit. When women compete for men, men win: the price of sex goes down."

The two authors bulwark this marketplace theory with stats. In 1947, for every 100 female college students, 245 male students were on campus. Today, for every 100 women, only 74 male students are on campus. Today, the likelihood that unmarried women will have had sex during the past month is 60 percent higher on a campus where seven out of 10 students are female than on one where only three out of 10 are female.

Is the sex-ratio theory only a theory? Regnerus and Uecker found that women on campuses where they make up a higher proportion of the student population "express more negative appraisals of men on campus . . . hold more negative views of their relationships . . . go on fewer dates . . . receive less (in the way of relationship commitment) in exchange for sex."

In short, good news for women's career opportunities is bad news for their marriage opportunities. A typical young woman, the authors say, is looking for "love, attention . . . affection, commitment, and feelings of emotional union": When men have less reason to make commitments, women go further to maintain a relationship than they otherwise would, so "an oversupply of women leads to a sexually permissive culture."

Regnerus and Uecker go beyond the data to critique the action of pandering college leaders. They note that "stories commonly used to socialize students about campus sex are skewed, creating a more positive spin on hooking up that is out of step with the reality of many actual student experiences. In a consumer-driven marketplace like the university, adults give students what they think they must want. What results, however, is reinforcement of the message that sex is . . . what counts."

What can be done? We can't depend on professors and administrators. Nearly two decades ago I made my one attempt to turn back the waves at the University of Texas by asking colleagues in the Faculty Senate to critique an administration plan to put condom machines in freshman dormitories. I got nowhere. I even offered the pragmatic stat that Regnerus and Uecker note: "Fifteen percent of couples practicing typical condom use will become pregnant within a year." I got nowhere.

A tightly held faith in God makes a difference. The authors found that those most likely not to be having sex are "more religious, especially in terms of how central it is to their identity." Apart from belief that they are called to marriage and fatherhood, men are unlikely to marry in their 20s, or perhaps at all: "Their decision to delay makes sense from a sexual economics perspective: they can access sex relatively easily outside of marriage, they can obtain many of the perceived benefits of marriage by cohabiting rather than marrying, they encounter few social pressures from peers to marry. . . ."