DK, it's a fallacy to think that human's are by nature polygamous. By nature we are actually monogamous. A male would be hard pressed to provide for more than one female and children. Just because the desire is there, doesn't mean it's practical.

Brit, the only example I could think of would be situations of extreme emotion, where instead of turning to each other for comfort a couple is forced to look elsewhere, and then realize after that this was the wrong thing to do. For example the death of a child. Both parents might be too busy blaming themselves to seek comfort in each other, and it might lead one to stray in a effort to find emotional relief, and then later realize what a mistake they made.

Also, situations like severe depression, or mental illness can cause someone to stray.