My my, lwb.. your thread certain provokes lively discussions!
Having your spouse escape to another's arms, heart mind etc is a betrayal like no other. Women must be hardwired to experience this action so deeply.
Now.. I like to think of it as parallel to a man's erection. When a guy sees a curvaceous creature oozing sensuality, licking her tongue slowly over her moist glossy lips while leaning over as the rounded breasts are revealed.. he comes to attention. He's born with a built in babe barometer.
Yet, he learns to manage his natural reaction with what's important and vital to his life. I tell you, I would never want a penis. At least women's sensual thoughts aren't as obviously revealed.
It's your choice how you deal with it. What is important and vital to your life? What will make you a better person. Getting mad makes sense, but then what? A life lived in anger is like asking for gangrene of the soul.
He's a schmuck for running away from the problems. We're all at fault for not addressing the issues. A good friend, a best friend being part of the betrayal means.. that person wasn't a best friend. The attraction was like water, seeping in, eroding resolve until the escape seemed so much better than the reality.
Remember, as someone wise told me. You are the stable one. Your spouse isn't. Stability comes from facing truths, realizing you can't (nor should you) control another's actions. Having a focus of perpetual anger and hurt damages you, far more than him or her. They have their own brambles to be shredded by. Take care of yours.