Some recently reported studies indicate that whether or not you cheat may be partially explained by genetics.
A study on Finnish twins and their siblings found that a presence of a single gene was linked to a significant variation in infidelity among women.
Incidentally, this gene appears to have no effect on promiscuity in men.
The idea that men have a biological and evolutionary impulse to cheat- because that could increase the chances of passing on genes to offspring – has been around for a longer time.
Many scientists maintain that cheating has never been "just about"procreation, and that it can be pleasurable as it triggers release of multiple hormones and activates the brain's "reward circuit."
These senses and memories may be enough to influence the person to cheat again.
In addition, humans are one of the very few mammals that practice monogamy.