It's human nature to want variety.
Monogamy is lifestyle (choice) not a DNA code.
It is a learned lifestyle along with various other things that fall under society's ideas of morality. It's human nature to want (your) way.
The word (cheating) means to "break the rules".
One of the roles of religion and government was to establish guidelines/rules for society. Generally speaking most laws don't come into existence until the group feels things have gotten out of hand.
People who are truly "in love" are more likely to be faithful because they don't want to risk doing anything that could cause their mate to leave them. Those who aren't "in love" (attempt) to practice "self-discipline". For them monogamy is like being on a very strict diet.
People cheat on diets all the time!
There is a big difference between someone (loving you) and being "in love" with you. The first one means you're not 'the one".