I believe there is so much distortion during an A with nothing being based on honesty and truth that it would be nearly impossible to judge.
I am sure it depended on the day. When they were on their affair high, I am sure the A was more important because they were making the conscious choice to be there and cheat. They put the A first over the M.
The A meant something at the time. It was validation, fulfillment, ego boost, whatever. Whatever it meant didn't make it right but I do believe it meant something or it never would have happened.
BOOM DDay = and there is a whole new reality. Fear of losing the M, the BS, the family. Looking at the A without rose colored glasses and seeing it for what it is; a shell of a relationship based on lies and deceit. Suddenly it isn't so appealing most of the time.
We all compartmentalize to make it through is some manner. That and I believe in most cases, the BS will never know everything.