Cheating

I met a couple recently who have been together for about 5 years. He's married, but not to her. His wife apparently has no idea. It's not really any of my business about these two individuals' personal choices, but it got me thinking about similar situations.

First off, do the wives of cheating husbands (or vice versa) really not know about what is going on behind their backs? I bet they have some idea, but are just turning a blind eye to it.

My other question is, if someone is unhappy, why wouldn't he or she just leave? Are you really doing your spouse a service by lying behind his or her back? Monogamy sometimes doesn't work. It's not a failure, it just happens to be the case sometimes. There happen to be many people who can have a completely monogamous relationship, but there are many who cannot. Is it really fair to string others along? I guess it just has to do with circumstances.