On some level I do believe in "judge not lest ye be judged". But on the other hand I am by nature a rather judgmental person..

So regardless of what any of us may think about the state of marriage, how desirable it is or what social/personal/financial/sexual purpose it serves I think that most of us would agree that lying to someone and deceiving someone is not a good thing. The very nature of having an affair requires deception of the spouse and self-deception. That's the essence of the problem imo.

There are some people that are not monogamous or are otherwise just not meant to be married. Some of those people have enough personal charisma or attractiveness to the opposite sex to be open about their non-fidelity. And of course people change over time. I can't judge any one else's personal life. But I will say that lying is wrong.