Michael Metz is growing less surprised by phone calls like the one he got a few weeks ago. A worried young wife said that the couple's sex life had taken a noticeable nosedive. "Is there something wrong with us?" she asked. Probably not, said Metz, a St. Paul-based marital and sex therapist. Any couple married for five years, with young children, should expect a dip in intimacy. Then came the punch line: She's starving for the hot sex they used to have. He's the one with the "headache."

A guy who doesn't want sex? We're kidding, right? Uh-uh.

Metz is one of many sex therapists noting a shifting dynamic in the bedroom, as women are finally talking about a problem many of their mates would rather shove under the mattress: erectile dysfunction and diminished libido in men under age 40, married and single. Studies suggest the problem is trickling down to younger, college-aged males; as many as one-fourth face performance problems.

Brian Zamboni, a therapist with the Program in Human Sexuality at the University of Minnesota, also has seen a "significant and steady stream" of new male clients, ages 20 to 35, in the past five years. One of several new books on the subject calls low sexual desire among men "America's best-kept secret." Some men, though, are sharing their frustration and confusion.

"Yes, I'm 25 and I have a sexual dysfunction already," posts "Z," seeking advice on a medical website. "I don't have erections in the morning anymore. I must sleep first to have sex later. Why is this happening to a healthy guy like me?"

There are lots of possible reasons. Too little sleep. Too much porn. Cigarette smoking and other unhealthy habits. Unresolved resentment toward one's mate. And it doesn't just affect the guys. Partners feel the impact, too.

"This morning, I was once more rejected and I started to cry because it's getting to me," writes a woman on the "Mismatched Libidos" message board, directed to people in their 20s and 30s. "I feel ugly, I feel fat."

Another writer shares similar frustration. "My husband would always say, 'You have an incredible sexual appetite. I hope you never lose that, especially when we have kids.' Well, two kids and 10 years later, I am not the one who has lost it. He has. There is always one excuse after another."

If there is good news here, it's that unleashing men from the myth that they always want sex allows women to silence the fiction that they never want sex. Metz, who has written three books on men's sexual health, believes that it is women's increased confidence in -- and demand for -- sexual pleasure and intimacy that is forcing this issue into the open. "Women are expressing more frequently that sex is important to them," he said.

For most men, sex is still very important, too, which makes the inability to perform difficult to face or fix.

So, what is going on?

Well, a lot. And this is part of the problem. "A good number of men, even young men, get exhausted," said Metz. "The competitive pace, the demands of work and kids, the decrease in leisure time -- it has to show up somewhere." But therapists and doctors see many other factors playing a part. Among them:

• His health. Is he drinking? Smoking? Alcohol and cigarettes both inhibit erections. Is it his prescription medication? Some antidepressants and blood-pressure drugs can lower desire or delay orgasm for men and women. Obesity, diabetes, sleep apnea and thyroid imbalances can all hamper sexual success, too. And what about illicit drugs? "The research isn't definitive," Zamboni said, "but cocaine, marijuana and heroin use can inhibit erections." Low testerone levels could also be a treatable factor.

• Psychological stresses. This is the "feelings" conversation many men just hate. But it's essential to draw them out if they hope to get to a solution. "A number of young men are lonely in their marriages," feeling like the third wheel when babies arrive, Zamboni said. Not just lonely. Some are silently fuming. Bob Berkowitz and Susan Yager-Berkowitz asked men why they stopped having sex with their wives for their new book, "He's Just Not Up for It Anymore." Forty-four percent answered, "I am angry at her." Men felt criticized and controlled, undervalued and insignificant, yet "many couldn't, or wouldn't, talk about it with their partners," they write. Thus begins a downward spiral. He loses his desire, starts having performance problems, starts to avoid sex.

• Her reaction. At this point, things may go from bad to worse. Women (and some gay partners, Metz said) aren't thinking, "Poor guy, he has performance anxiety. I need to be patient and affirming." She personalizes: "He doesn't love me anymore! He must be having an affair!" Not surprisingly, she pulls away. Sometimes, in fact, he is having an affair, or she is. Sometimes, though, women say his "affair" is with pornography.

• Pornography. Is pornography a culprit? Maybe yes, maybe no. There's certainly a risk, Zamboni said, that a man can become dependent on porn. "Instead of going to his partner, he goes to the computer." There's also a risk that his view of normal women's bodies and capabilities could become unrealistic. Consider this blog entry on Cribsheet (startribune.com/cribsheet), a Star Tribune forum for new parents. "I'm sorry to have to say this, because it must be painful for a woman to hear," writes a young father, "but I just don't find my wife as attractive as I used to. Pregnancy takes a toll on a woman's body and -- I'm sorry -- I'm just not turned on the way I used to be. If that makes me a cad or a superficial jerk, I'm sorry. But there it is." (Never mind that he, too, has likely put on a few, or more than a few pounds. Men don't tend to see that as a problem. Sorry, but there that is, too. This is probably a good conversation for couples to have when they're both feeling well-rested and collaborative.)

But Metz also knows some couples "who use traditional porn for their own sexual energy. Porn is about fantasy," he said. "Fantasy can be fine, if you know what you're doing." How can anyone be sure? Metz encourages couples to ask a very sensible question: Are your actions around pornography encouraging intimacy with your partner, or hindering it? "A man who masturbates four times a week to porn and is ignoring his wife," Metz said, is clearly compromising intimacy. "If he does it one time a week as a reaction to a high-pressure career and it's a tension release," the act may not have any ill effects on the couple's intimacy, he said.

• Unreasonable expectations. We live in a world where erections are considered problematic only if they last more than four hours, and women feel inadequate if they don't own several lacy thongs. No wonder people are shell-shocked. We also think everybody's doing it with more frequency and flair than we are. Probably not. Still, everybody's anxious. That's doubly true, Metz said, for other cultural groups. He's counseled couples and men in the Muslim community, for example, for whom talking about sexuality is "a shy, cautious experience. I see it as a double edge," he said, "... the deep respect for the body and private intimacy, [coupled] with an anxiety, especially for men, who commonly have premature ejaculation. The difficulty of talking about sex is similar to other couples."

Metz wishes that everyone would follow a "good-enough sex" approach, where pleasure becomes as important as performance. That means relaxing and touching. It means taking the pressure off by finding other ways to enjoy intimacy. It also means getting real about what's reasonable.

The woman who called Metz eventually revealed that, while their sex life had diminished, they were still making love once a week. That, according to many studies, is about average and certainly no cause for alarm. "You're not going to be having college sex when you're raising kids and working your butt off," Metz said. "The crucial issue is being realistic about expectations. How sad it is that this pursuit of 'perfect' sex becomes a cause of sexual problems."

Gail Rosenblum • 612-673-7350