Playing devil's advocate, why can't we be more accepting of infidelity if it allows the marriage to grow. I've always believed in monogamy but the number of guys who cheat on their girlfriends, wives, etc. is high. It makes me think of that quote from JM Coetzee's Disgrace - I'm paraphrasing here but it's something like: "Animals won't accept being told not to do something which is in their nature."