You and I did not come from monogamous ancestry. We came from folks who enjoyed fucking - a lot. Otherwise, we wouldn't be here.
It's really simple to understand. The monogamous couple, who slept only with one another, who only had children with one another - and lived together in monogamous bliss - simply didn't survive. One virus, one cheeta attack, one flood, one cold snap could wipe out the entire family. End of that story. End of lineage. Where as, the ancestors who inadvertently diversified their gene pool by having sex with lots of different people, in different locations, over their entire lifetime - ensured the survival of their lineage.
We exist because our ancestors had lots of sex with lots of different people. Different combinations naturally create a stronger humans. Evolution predicts that that we will inherit the sexual proclivity.
It is in our interest to mix it up. It's not in our genetic best interest to keep inbreeding - like so many religions preach.
Diverse sexual attraction is a daily part of life. Our very survival depends on hooking up. In fact, we could argue that without attraction and hooking up, there would be no human race. Our entire existance relies on the reality that people want, need, and desire sex with lots of different people.
In my practice I have found that most everybody has some kind of relationship connections outside their primary relationship. I avoid using the word cheating, because cheating presupposes a valid underlying truth, correctness - an absolute REALITY about monogamous relationships - which clearly is a made-up, mythological idea.
I also find a tremendous amount of myth in the emotionally charged word infidelity. Similarly, it presupposes an underlying spiritual monogamous reality that underpins this reality - a monogamous relationship to god. It's the stress of "the right road", the non-sexual road, the religious road, versus the reality of sex, attraction, and propagation of life that is life.
Sex is life. Life is sex. Monogamy is just another story.