My XWW was super moral when it came to cheating. She literally dissolved a friendship, when she found out that friend was sleeping with a married man, with children. I knew she would never, ever even consider cheating.
When she did, she had done mental gymnastics to convince herself she was justified. In her mind, I had become a boring person (I am an executive with an active social life, hobby musician, artist, writer, furniture maker) and we had fallen into a routine that bored her. Nevermind that I brought her coffee in bed every morning, greeted her with "Good Morning, Gorgeous!" sent her love notes throughout our 18 year marriage and did the majority of domestic duties. She would cite that she would come home from work and "there you are, on the couch, eating cashews, drinking a beer and reading the New Yorker!" as though this was a damning indictment.
When discovered, after five months of telling me, in detail, how she figured we were probably done as a couple (those damn cashews!), I found out. She was ashamed, but like other cheaters:
She thought she wouldn't get caught
She thought she deserved the excitement
She wanted to feel desired by a new man
It was a cry for help
and now that we divorced, she blames it on me for over-reacting to something she considers "a stumble."
Clearly, she did not know the magnitude of the affair and still doesn't.