I don't think the retroactive reasoning has much to do with A's in particular. It is standard in most any M that breaks up that one person leaves. The person who leaves has a hugely skewed toward the negative view of the M. The person who was left gets a hugely skewed positive view of the M.
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.
