Cheating is the easy way out for a spouse who is afraid to make a choice about a current dilemma. Infidelity allows a person to ignore those tough choices and actively participate in an illusion that only brings temporary relief to a symptom, not the underlying disease, and in itself can become its own destructive force (in which case, the cheating spouse never did have to make the tough choice. Being found out makes it for them).
Divorce, in most cases, is the same thing without the lie, but a least those who seek a divorce (without the cheating) have the balls to own the fact that they just can't take it anymore and are honest about it.
