Many men (and some women) enter a marriage addicted to porn, for example. Doesn't matter if wife is a cute little babe and great in bed. The problem is inside the man. And then, porn just isn't enough for some men. The lure of the illicit....the forbidden is just too great, and some eventually succomb.
What we are seeing in the church is men who are hitting rock bottom with one-night-stand type affairs, confessing, and finally dealing with their porn addiction. Some, fortunately, begin to deal with the addiction before they hit bottom.
There are a variety of reasons why spouses "cheat"; just pointing out that dissatisfaction with the marriage, or spouse, is only one reason.
I agree there are various reasons for married people to have affairs. People can always come up with a reason, or an excuse. What I see is people who cannot keep their marriage vows. The results are the same regardless of the reason. The hurt and the damage are still there in the marriage after the affair(s). And that would seem to be the bottom line.