Men are designed to deliver DNA, they are like love letters from one female line to another. This is necessary to keep our genetic diversity robust, and thereby ensure the survival of the species.

Most men, as a result, like a number of partners, and many women feel the same. It is healthy for the species as a whole, but not that much fun to deal with as an individual living in a "nuclear family" based society. In societies where the descent is Matriarchal, and the child-rearing communal, and the mating usually outside the tribe or extended family, it is a normal and natural and health thing to have more than one partner in a lifetime.

Men "cheat" because we have expectations and labels that define natural behavior as "cheating". If we acknowledged that many of us do not expect to be monogamous, there would be much less upset when we are not.

The reason why most men/women cheat is an inner self sabotage they get to a point in themselves that when all things are going so amazing and then "BANG" the inner sabotage emerges, most likely from some kind of child hood connection to when someone or something they had or did was taken away just at a point of feeling fulfilled and happy with it and it was taken away so they go around trying to replace or fulfill that of which is not there jumping from one thing to another and not aware that the only thing and one they need to know is that they are fulfilled just the ego that emerged at the moment that thing or person was taken from them believes they are only fulfilled if they have this something outside of them so they go around looking searching and taking this energy or object from people getting their fill until they are satisfied and happy until the next time the sabotage mechanizm happens again. So the spiral of the energy vampire in them continues going around taking from others due to they are not aware that they all ready have the energy and full fulfillment in them so they continue stealing it from others.

Kinda like a thief or a rapist they only take that of which is not theirs due to they do not have that sexual energy or materials themselves so they go around stealing it from others to fill themselves up each time.