Not that it’s an excuse for cheating but it turns out that there is a strong chemical component to infidelity. This is partly due to the Coolidge Effect which is named after American president Calvin Coolidge.
… an old joke about Calvin Coolidge when he was President … The President and Mrs. Coolidge were being shown [separately] around an experimental government farm. When [Mrs. Coolidge] came to the chicken yard she noticed that a rooster was mating very frequently. She asked the attendant how often that happened and was told, “Dozens of times each day.” Mrs. Coolidge said, “Tell that to the President when he comes by.” Upon being told, President asked, “Same hen every time?” The reply was, “Oh, no, Mr. President, a different hen every time.” President: “Tell that to Mrs. Coolidge.”
The Coolidge Effect essentially states that men’s ability to get it up for a second round is greatly increased if that second round is with a new partner. Basically men (and to a lesser extent women) are more easily aroused by women or men that are not their primary partner. Kinda sucks for monogamous people when you think about it. Cheating is kinda built in to the equipment.
Anyways, this excerpt from the book “The Chemistry Between Us: Love, Science and the Science of Attraction” briefly explains the chemical component of cheating.
Human beings are born with a bias toward satisfying immediate rewards, and our ability to counteract that bias in the service of a longer-term goal — like preserving a bonded relationship — when we sense something (sexual opportunity, for example) that sparks our reward circuits, may depend on the strength of the interaction between those circuits and the PFC (Prefrontal Cortex).