It is often when one spouse moves toward a more moderate (and realistic) view of the M that the other spouse does as well.
And, a realistic view almost certainly includes the fact that the M was not good for either of the partners for a very long time, long before any A. The A is indeed a symptom, not the cause of the dysfunction in a painful M.
I think one of the distinctions, at least in my mind, is between the marriage as a whole -- which as you say, oldtimer, is often dysfunction at worst, or at least had some major problems at best -- and the IN LOVE feeling of the partners toward each other. A wayward spouse often re-writes marital history to say that "I never loved you," or "I haven't loved you for YEARS," when it's simply not true -- it's the brain rush of the new paramour skewing their view.
And of course a large number of affairs, as documented by Dr. Harley and others, happen to even HAPPY marriages.
