Actions aside, 71 percent of people say it's never OK to be unfaithful. Yet, one in four men and one in 10 women think cheating is justified if a partner has no interest in sex.

“People who engage in marital infidelity think they have a good reason, but this is an area where our behavior doesn’t fit our attitudes in a very large way,” says Howard Markman, a professor of psychology and co-director of the Center for Marital and Family Studies at the University of Denver. “People are amazingly adept at justifying their negative behavior; it's one of the biggest problems in marriages.”

About two-thirds of cheaters say they don’t regret their actions, and 12 percent of men and 13 percent of women say they’re glad they cheated.

For many "it was a life experience, or a daring adventure," says Lever, the survey's lead researcher.  "They had some fabulous sex for a week and they didn't regret it."

But many did face lingering feelings of sadness (25 percent), stress (32 percent) and guilt (49 percent).

"The only thing that turned out from cheating was feelings of guilt and shame," writes a 31-year-old woman who is currently single. "It most definitely made me realize how much I loved my primary partner and that anyone else was not worth it!"

No doubt infidelity is a serious problem that often leads to divorce or damaged relationships — 19 percent of people who were cheated on ended the relationship right away and 22 percent eventually broke up because they couldn’t get over the betrayal. Sexual infidelity played a role in just over half of divorces, the survey found.

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"The fallout from affairs is not as much fun as the fling," says Leiblum. "When affairs come to light, the damage to the relationship is quite substantial. It can take months and even years to lessen the toxic effect of disbelief, anger, hurt and betrayal and even then it’s not totally gone."

A 29-year-old woman who has been on the receiving end of such a betrayal agrees. "When someone cheats on you, it destroys your self-worth."

Love keeps us true
What about the true blue among us?  What motivates those who stay faithful? It’s not lack of opportunity. Only 8 percent of men and 4 percent of women say they’ve never had the chance to fool around.

For the most part, love does keep people faithful. While 68 percent of men in a monogamous relationship say they've desired someone else and 43 percent of women have had the hots for another person, they're not lighting their fires with someone else's match.

More than three-quarters of participants say they are too much in love to be unfaithful and 68 percent don't want to risk losing their partner. Love of one's partner was also one of the main reasons why people stopped cheating (20 percent).

Even among couples that have been together for more than 30 years, four-fifths of women and two-thirds of men report being faithful during the entire relationship.

For some, remaining faithful is the ultimate symbol of dedication. "She is the love of my life," writes one 31-year-old man about his wife. "I searched years to find her and I would never want to ruin what took so long to find."

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