Oh do you want the short answer or the long one?
I will, I promise, I am a bit busy at work at the moment, but stay tuned
I am amazed at the number of women that are really clueless, I mean no disrespect.
Basically I would say it's either to stroke his ego or he's unhappy at home or work and wants to escape for a while. Some men can't control their urges when it comes to women (which is scary).
I don't think that it's the wife's fault unless she is nagging him all the time and makes him feel like he's worthless, whether she realizes it or not. That would probably fall under the communication issue that Mami was talking about.
Because they are just stupid! It's the thrill of the opportunity and thinking with the wrong head. Someone who cheats, is selfish, self serving, opportunist, like's the thrill and attention of 2 women, lack moral values and respect for their partner. Don't realize that they are hurting the very person they love or no longer love. Some men turn to other women when they are unhappy in their marriage or relationship.