Years ago I used to work as a counselor. I have some other credentials I could tell you about privately if you want to Freepmail me. This is what I heard from people who came in for counseling and from the records of others I read. No, I'm not making it up.
Most of the time I didn't hear the guilty party (male OR female) say that things were terrible in their marriage. If they did have complaints, they were such stupid, trivial complaints--my rotten husband works all the time and then actually has the nerve to want me to go to bed with him, my wife never lost 15 pounds after the baby was born and now only wants to have sex three times a week instead of seven, that kind of thing. Small things people could work out if they weren't selfish. It was usually just that the marriage kinda boring, not quite what they'd hoped, and this new person was so hot and they had opportunity, so they didn't really try to resist the temptation. And then they were caught and were sorry when they saw the seriousness of the matter.
I was areally struck sometimes by the fact that people could be discontented by spouses that anyone else would give an arm and a leg to have--beautiful, sweet, intelligent, accomplished, supportive people. But the other party could not have been pleased by anybody for long and would always find things to whine about.
Please remember this isn't an attack on men or their morality. Some women are unfaithful too, and I heard the same things from them. I know there are occasions when a marriage was ghastly and a husband stayed with his wife to prevent the kids from being abused or taken away from him, and then had an affair, but this wasn't what we routinely heard.