Wild title!
Since I'm hardly qualified to judge the entire human race, I'll limit my answer to me (!) and my wife (!!) and ex-wives (!!!).
1. Me. Did cheat once, toward the end of my first (of 7) marriages. Reason: Flailing around emotionally, angry at wife, bitter, whole host of negativity goin' on.
2. Wife #2: Did once, early in the relationship. I hadn't quit trying to be a control freak (though I was working on it), and it was her way of saying, "Oh, no you don't!" Which I understood. Painful, sure, but got the point across.
3. Wife #4: Did once, late in the relationship. Her way of trying to sort of "push me out the door".
4. Present wife: Did with 15 to 30 different partners, early on. Reasons: (1) Hadn't bottomed out on alcohol yet, (2) Hadn't yet figured out (after being molested by her father) that she COULD say no. Note: That all went down during our first 2 years together. None since, and we've now been hooked at the hip for 13 years and counting.