When one of the two or both cheat

"Some sociologists have argued that 'being faithful' is the central, defining norm of marriage," said Paul Amato, a professor of sociology at Penn State. "Although marriage implies multiple obligations, the obligation to be sexually faithful to one's spouse seems to carry the most weight."

The prevalence of marital infidelity and extramarital sex varies widely depending on the definition of infidelity used and the survey referenced, ranging from about 10 percent of couples to more than half.

A 1994 study by sociologist Edward Lauman found that 10 percent to 11 percent of spouses had cheated in the prior year. Over a lifetime, that study revealed about 18 percent of women and 24 percent of men reported an extramarital affair.

While Americans have become much more accepting of premarital sex during the past several decades, they still view extramarital sex as somewhat intolerable, Amato said.

When deciding whether to go the divorce route or follow the winding roads of marriage-repair, many factors come into play. In addition to cheating for different reasons, men and women react differently to an unfaithful spouse.

"Typical reactions from both sexes include becoming enraged, sad, humiliated, and depressed," said David Buss, a professor of psychology at the University of Texas at Austin. "There are large individual differences within each sex; men tend to focus more heavily on the sexual aspects of the infidelity; women more on the emotional aspects."

These differences may have deep evolutionary roots. "From a man's perspective, sexual infidelity historically jeopardized his paternity certainty -- 'mama's baby, papa's maybe,'" Buss said. "Male sexual jealousy is, among other things, an adaptation designed to solve the problem of genetic cuckoldry."

Women, on the other hand, are 100-percent certain they are the mothers of their children. And the most upsetting acts of infidelity from a female perspective involve the emotional ties their husbands may have formed with the significant or insignificant others. They are more likely to forgive their husbands if the affair "meant nothing" and involved no emotional intimacy. Overall, women are more likely than men to forgive a cheating spouse.

"So one-night stands and use of prostitutes is less threatening than is a long-term, emotionally bonded extramarital relationship," Amato told LiveScience. "Wives are more likely to forgive their husbands if their husbands were not 'in love' with the other woman."

"Women are more likely to take into account their children, their economics, their general survival," Schwartz said. "Men are just crushed or upset about what happened to them. They won't think as quickly about their children as the first or second issue; but they will eventually consider that."

"Men are less willing to forgive," said Ruth Houston, founder of www.InfidelityAdvice.com and author of "Is He Cheating on You? - 829 Telltale Signs." She added, "Men view infidelity as a statement about their manhood, so it's such an affront to him that most men cannot get over this hurdle."

"Wives are also less likely to consider divorce if they are economically dependent on their husbands, have children or hold strong religious views," Amato said. "Nevertheless, most wives at least consider the option of divorce. And, in fact, infidelity is the marital problem most likely to lead to divorce."

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