She’s Gotta Have It

Some experts, such as anthropologist Helen Fisher of Rutgers University, theorize that the mellowing may be due to certain brain chemicals that come into play during three distinct relationship phases: lust, romantic love and attachment. "That's not to say that passion goes away entirely, but it certainly wanes," says Yager-Berkowitz.

The couple also believes that the complaint that a woman is "sexually unadventurous" may be a code for other problems, such as a husband who doesn't do anything a wife finds enjoyable or because he may feel underappreciated. Apparent indifference can also be a cover-up for other problems, like erectile dysfunction or depression.

Despite the dearth of intimacy, most women and men surveyed say they aren't thinking about divorce. The rationale: sex, or lack thereof, wasn't an important enough reason to end an otherwise fulfilling relationship. But what does trouble experts is that this lack of sex generally masks some other issues that will eventually rock the foundation of a marriage—and for the woman, feelings of rejection can be devastating in the long term.

Michele Weiner Davis, author of the newly released "The Sex Starved Wife: What to Do When He's Lost Desire" (Simon & Schuster), has also found that women "don't have the corner on low libido." But men, she says, have difficulty opening up about their lack of desire, making it tough for couples to work on a solution. "Even though men have been told for decades it's important to be in touch with their feelings, men have a terrible time talking about what it feels like to be less than a stud," she says.

Anger is often an underlying issue contributing to low libidos among men. According to the Berkowitzes' survey, more than 40 percent of men said they're angry at their wives. That anger can stem from a rotten job, feelings of inadequacy and desire-busting problems like having a partner who focuses on the negatives in a relationship. "Rather than standing their ground, men can fall into the pattern of a constant simmer that eventually boils over," Weiner Davis says. "It's a classic pattern that's deadly, especially in killing desire."

What to do? The conventional wisdom is that if a married couple is happy not having sex, it's all good. But Weiner Davis doesn't buy it. "I believe that sex and touching is a tie that binds," she says. "Sex isn't like vitamins. There's no minimum daily or weekly requirement. But human touch is important."