I think the major reason, is simply their needs are not being met by their spouse.
But I often see, the wives less of the culprit (especially those with children.) - from my observation, in many cases its the wife that is actually the unhappy one. Her husband is failing to meet her needs over a period of time.....causing her to feel angry & withdrawn. The husband may be tired of being confronted about same issues? pressured to possibly change? making him feel less of "her hero!".....as a result he finds it easier to leave the drama/ or thick silence to be with another desired companion that makes him feel excited!.....makes him feel like a wonderful man "just the way he is" and sexually inviting again.
*Perhaps its just pureSelfishness in this day and age, its happening at an alarming rate.