My DD was asking me about this kind of thing, and my response to her was that it's all about the agreements couples have--sometimes they're even unspoken agreements.
So, if it understood that a relationship is open, and each partner plays around, so what? It is, by agreement, an open relationship. The problem is when the relationship has been built upon monogamy, and one cheats, then it is a betrayal of trust. So it's not the act, it's breaking of the agreement that matters.
I was watching the show on polygamy, and that's a case in point. Most of us could not FATHOM sharing our husbands with multiple other wives, but if the wives have not been coerced, and are happy in that agreement, I wonder if it's wrong....
I also read about Rose Kennedy, who knew full well what kind of a man Joe Kennedy was, and when she realized she couldn't rely on him for a couple of aspects in their marriage, she resorted to her faith... and daily spa treatments.