Men have been cheating for thousands of years. I would chalk this up to our instincts for self-preservation being more finely honed than women’s.
But who are these men cheating with? i bet they all aint single
The successful reproductive strategies of men and women are different. Men want to spread their genes by impregnating as many women as possible. Women want to have a man around to help protect and raise children, and they want the best possible man to impregnate them (for healthier children).
Mating for extended periods of child rearing is what enabled human beings to rise to the next level of advancement. Marriage was developed as the formal means for enforcing that mating constraint on men, who otherwise would be still trying to impregnate as many women as possible.
Marriage is a relatively recent invention. "Cheating" is a recent interpretation of otherwise natural instinct, layered over the biology which has been around for hundreds of thousands of years. Biology does not change quickly.
So, the reason men run around is obvious. As to why women run around.... they're usually looking to get themselves pregnant by a better man than the one they have (assuming they have one). Again, this is biology, and does not change quickly.
Here's the REAL kicker: Our modern welfare system has completely screwed up the natural instincts, by taking away the loyal male mate and replacing him with the government handout. It is a travesty.