Why do married people cheat?

It could be as innocent as 'I wonder what else is out there.' Not to hurt the spouse, they put aside the reasons for marriage. Marriage used to be because you found the one you could spend your entire life with, only. Then, a casual aquaintence sparks you on any given day, and you forget the special thing that keeps your marriage sacred. Sometimes your spouse finds out, doesn't tell you, and plays the same game; no harm no foul.