(LifeWire) -- He's a 38-year-old executive. She's a 34-year-old homemaker. He says they never fight, and in many ways they're compatible -- but not when it comes to sex.

"It's almost like a checklist," says Jon (who asked that his real name not be used) of their once-a-month lovemaking. The problem, he believes, is a lack of desire.

Sexually unfulfilling marriages aren't limited to new parents or aging baby boomers with hormone imbalances. They can ensnare even the relatively young and the recently married. When they are unable to blame kids, stress or physical issues, many couples struggle unhappily to identify -- and resolve -- the problems behind their lackluster sex life.

Couples end up in sexually unfulfilling marriages for a variety of reasons, says Marty Klein, a licensed marriage counselor and certified sex therapist in Palo Alto, California. One reason, he says, is America's obsession with marriage.

Laura Berman, a Chicago sex therapist and relationship expert, agrees. "We put the blinders on when we're dating," she says. "We focus so much on the wedding, we don't notice the warning signs."

Those who believe passion inevitably fades may downplay the sex factor, picking someone they think would be a good father or a good wife even if they're not an ideal lover, Berman adds.

"I chose her because I thought it would enhance me in some way," Jon says of his wife.

Berman has seen it before: "People choose partners who have the right resume but maybe not the entire package."

Other couples enter into relationships with so-so chemistry because they think they're in love and overlook key differences, says Klein.

Bobbie Jonas, a holistic health practitioner in Calistoga, California, acknowledges she ignored obvious warning signs during her courtship. "I was more interested in a way out from home," she says of her first marriage. Poor communication compounded the effects of weak chemistry. After 10 years, they divorced.

"Couples wondering where the sex went should be asking if it was ever really there," says Berman.

That explanation makes sense to Jon. Although he said he and his wife, who live on the West Coast, started off with great chemistry, the cracks in the relationship began to show before they traded rings. After a four-month dry spell during their engagement, his wife brought up the idea of canceling the wedding. "I just really wanted to get married," Jon says. "I felt that it was what I was supposed to do."

Now Jon is having an affair with a woman -- also in a sexually unsatisfying marriage -- for whom he feels intense passion. "I didn't realize the importance of sex," he says.

It's not always a problem

On average, Americans report having sex 85 times a year, according to the 2007 Sexual Wellbeing Global Survey conducted by Harris Interactive for Durex. The largely online survey polled 26,032 people in 26 countries using random samples of those aged 16 and older.

Therapists generally define "sexless" marriage as having sex less than about 10 times a year, and they estimate 1 in 5 couples are in such a relationship.

But Klein cautions against looking for problems where there are none.

"A dry spell is only a problem if the couple thinks it is," he says. "There are plenty of couples who don't have sex and don't think there's anything wrong with it. And there's others that are in a lot of pain about it."

Klein notes that the expectation of eternally passionate sex may be setting people up to fail. "People have the assumption that you can have long-term, monogamous, hot sex," he says. "It's never been done (on a large scale) in the history of the world."

Getting more sex

Berman offers at least one reason to resolve unsatisfying love lives: "Often, when you're not having sex, your empathy and ability to connect is lower, and it's easier to have conflict," she says. "It amplifies (marital) problems."

At the Berman Center in Chicago, she counsels couples on repairing their sex lives. Some advice:

• Try traditional gender roles: Men may become more sexually assertive if they feel more in control, and women may feel more desire for a mate with newfound machismo. "You don't have to get his slippers," explains Berman. "You just have to give him some control." She suggests a date where the man chooses everything -- her clothes, the restaurant, the food -- as a starting point.

• Engage in exciting activities: Whether it's trying an extreme sport like skydiving or snowboarding, or exploring new options in the bedroom, activities that get the pulse racing can open the brain's dopamine centers and increases desire.

• Talk about it: Couples also would benefit from simply communicating with their partners about what they want in bed. "There is no secret to hot sex," says Klein. "Sexy lingerie and dinners out are no substitute for an honest conversation about sex." E-mail to a friend

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