MSNBC.com/iVillage Lust, Love and Loyalty survey results

The obvious answer to the first question is sex. Men and women both cited a strong physical attraction, and men especially cited a craving for more sex and sexual variation, as reasons for straying. And while most people — 70 percent — say cheating is never OK, of those who felt it could be justified, the top rationale picked was a partner losing interest in sex (18 percent).

But "sex" is complicated.

Research into why people seek sex outside their committed relationships has been going on for decades, but experts still don’t think they can say much of anything with certainty.

Battling biology?

University of Washington sociologist Pepper Schwartz, author of "Prime: Adventures and Advice About Sex and Love in the Sensuous Years," thinks the very idea of monogamy explains why some stray. Biologically speaking, she says, human beings aren’t built for it.

"I think what most of us say is, 'I want to make myself precious to someone. I do not want to lose him or her. I do not want to fight. I want to follow my religious teachings.' But these are cultural, religious or practical reasons for being monogamous, not biological," she says.

When we fight biology, we often fail to live up to our own ideals. "It’s like how people say, 'I am always on a diet, so why I am still so fat?’" Schwartz says.

Our ideals can be pretty strict when it comes to fidelity. Majorities of both men and women consider sending a sexually flirtatious e-mail cheating. And 21 percent of women say even sexually fantasizing counts as cheating; only 12 percent of men agree with that.

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