I can't remember who said it but, "..the last refuge of despair for a man is another womans arms.." The point is that many men who cannot make the break from a relationship they don't want to be in, find another woman. In a way this deflects the wife's rage towards the "other woman" and also give themselves a reason for leaving. Not an excuse, but really what people do in real life without really understanding what they are doing and why.
In addition, sometimes people marry for the wrong reasons or when they are so immature and insecure they really slip into a relationship with little or no thought as to why and to whom they are marrying. In times past, most died before 50, but now we live much longer; hence, marriages sometimes end when life used to end.
Also deflects societies rage -- how do men duck blame so easily?
So very true. I am glad you posted these thoughts. In the not very distant past I went through something very similar and my first reaction was to hate "the other woman". When I woke up and smelled the coffee I realized she was not the one who had lied and betrayed me. They are married now and I am very happy to report are both totally miserable. It's nice to see people get what's coming to them. Instant justice is a rare thing to witness. Over all my life is better without him but I wish to heck he had just address the situation directly. It's the lies that do the long term damage, not the breakup.