http://well.blogs.nytimes.com/2009/06/03/when-sex-leaves-the-marriage/

Why do some couples sizzle while others fizzle? Social scientists are studying no-sex marriages for clues about what can go wrong in relationships.

Married men and women, on average, have sex with their spouse 58 times a year, a little more than once a week, according to data collected from the General Social Survey, which has tracked the social behaviors of Americans since 1972. But there are wide variations in that number. Married people under 30 have sex about 111 times a year. And it’s estimated that about 15 percent of married couples have not had sex with their spouse in the last six months to one year, according to Denise A. Donnelly, associate professor of sociology at Georgia State University, who has studied sexless marriage.
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Why does a marriage become sexless? Does it start that way? Or does sex fade?

The answer to that one is both. Some of the people in our sample never had much sex from the beginning, while others identified a particular time or event (childbirth, affair) after which sex slowed or stopped. Some people become accustomed to their spouse, bored even, and sex slows. For others, it is the demands of raising a family, establishing a career, and mid-adulthood. And there are people who have very low sex drives, and may even be asexual. They may have some sex with their partners to begin with, but it becomes unimportant to them (and usually not so unimportant to their spouses). These folks may also be dealing with guilt, issues with the human body, or feel that sex is “dirty” or only for procreation. A small number of couples showed a mixed pattern, where they would have periods of “feast” and of “famine.”

Are couples in sexless marriages less happy than couples having sex?

Generally, yes. There is a feedback relationship in most couples between happiness and having sex. Happy couples have more sex, and the more sex a couple has, the happier they report being. But keep in mind that sex is only one form of intimacy, and that some couples are fairly happy (and intimate) even without sex. In my 1993 study, I did find that people in sexless marriages were more likely to have considered divorce than those in sexually active marriages. There is no ideal level of sexual activity — the ideal level is what both partners are happy with — and when one (or both) are unhappy, then you can have marital problems.

Read more at the nytimes: http://well.blogs.nytimes.com/2009/06/03/when-sex-leaves-the-marriage/