The closest cousin to humans among the great apes is the bonobo, whose behavior suggests that early humans enjoyed sexual relationships with a number of partners, even when involved in a primary, long-term relationship with a favorite partner. Men and women are believed to have had several sexual partners. Sex creates bonds that made small communities tighter. Women benefited because several men cared for their well-being, leading to increased food security and protection. Men benefited because their uncertainty about paternity meant that several men had an investment in the children of a shared sexual partner and would provide for those children.
Although human biology supports the theory that humans evolved to have multiple sex partners, humans are not constrained by biology alone. People can choose to remain monogamous and are accountable for honoring that choice once it has been made.
One study published today by the University of California, Los Angeles Center on Behavior, Culture, and Evolution and the University of New Mexico says women have evolved to cheat on their mates during the most fertile part of their cycle, but only when those mates are less sexually attractive than other men. The study in the Journal of Hormones and Behavior examined 38 coeds from one large, unidentified U.S. University. "We found that women were most attracted to men other than their primary partner when they were in the high fertility phase of the menstrual cycle," said Dr. Martie Haselton, a UCLA researcher. "That's the day of ovulation and several days beforehand." A related study, which will be published in Evolution and Human Behavior, finds that women are more likely to fantasize about men other than their mates, but only when they don't consider their mates to be particularly sexy. That UCLA study examined 43 normally ovulating women.
I think that humans are inclined to want to reproduce with as many people as they can, and that it is all in their nature. This is why people want to have sex with others besides just the one that they are married to, even though they made a life commitment.
Yes, humans are genetically inclined to cheat on their spouses, but that doesn't mean that it is right. It is natural, from a biological standpoint, because biology says to live as long as possible or reproduce as much as possible. But living an ethical life that honors a person's spouse is about not cheating.
As a human we are not genetically inclined to cheat on our spouses. We get used to the same old and what people want is something new and refreshed. If someone cheats they usually forgot the comfort and reason they fell in love with someone. It's more about self obessession rather then genetics. We all want to be wanted and in marriages sometimes we forget how much the other person really wants us.