Anthropologists have shown that humans are actually genetically programmed to cheat. Cheating it turns out was actually beneficial for human survival because in the time of memorial having genetic variety ensured survival of the species. Because of so many different diseases and, and such high infant mortality, the more genetic variety one could ensure the higher the likelihood that your genes would be passed on to future generations.
For that reason, men would often seek out as many sexual partners as possible because it would ensure that they would have as many genetically different babies as possible. Women on the other hand have a greater investment into their offspring between the pregnancy, nursing and so forth it can only have one offspring at a time and therefore, they wanted to ensure that their partner would stick around for a little while at least, at least four years to help them raise, that, that baby.
So for that reason, men and women developed different propensities for cheating. Men cheated for genetic variety. Women cheated for two reasons. One was paternity confusion because the more men they led to believe that the offspring could be theirs the more protection and, and the more resources they could have. That is if the ancestral woman would sleep with a number of cavemen, all of them would believe that the baby could be theirs and all of them would protect her and give her piece of the mammals that they caught.
For that reason those women who cheated were most likely to survive and pass on their genes. Women also engaged in infidelity for what anthropologists called resource extraction and that is to get things from men that they could not themselves obtain such as meat which was the primary commodity that men could obtain much easier than women. So for that reason our ancestors, the ones that survived, the ones that actually were able to pass on their genes were the ones who were unfaithful.
And so as human beings we're constantly fighting this ancestral evolutionarily [SP] programmed drive to cheat on our partners. But of course genetics doesn't mean destiny and evolution doesn't mean destiny because now we developed a more evolved brain. We're able to control our urges and we're able to commit ourselves to stable non-cheating relationships.