Why would he stay with her?

It's certainly a valid question to ask and wonder about. All I can think of is that he was so madly in love with his wife(which is completely understandable when she looks exactly like Diane Lane) and had such intense feelings for her that he, ultimately, just couldn't let go of her in spite of everything she did to him and their family. I mean I personally couldn't have stayed with her like he did(yes even if she did look exactly like DL). I simply could not have forgiven her for all of the selfish, hurtful and inconsiderate acts that she continued to willingly participate in with some random guy who just happened to have the ability to get her all hot and bothered between the legs.

So I don't know if my own decision(s) makes me any better or worse of a man than he was/is. But I do know that I simply could not continue to go on living with a wife who displayed such an utter sense of contempt for me as both a husband and as a person in general.

"Life IS pain highness. Anyone who says differently is selling something".

You answered your own question, "...who cherished his family above all".

There are a lot of unhappy marriages in which the two stay together for the sake of the children. In this case the fact that these two share a secret that's about as dark as you can get may make them feel like neither can leaves. Idk how something like this would weigh on the ol conscience, thank goodness.

You just answered your one question. "He's hard working, faithful and a great family man. He knows that what's best for his son is greater than his immediate response of anger towards his wife.

Plus although being unfaithful she isn't a horrible person and could over time mend the hurt she caused.

I was sympathetic to Gere’s character, up until the point when he murdered Oliver Martinez. After that, if I was Diane Lane I would’ve left him. How could she remain with a brutal murderer??