Why is it though that they affair down? Any theories? As I know there is no real answer.
This may sound too far out, but think of the wayward state of mind as if it her alter ego. That alter ego is trying to escape from her reality (which is personal unhappiness, issues with age, low self-esteem, problems in the MR, etc.). Her alter ego is not her true self.
If the OP responds or pursues the WW, then the OP's character shows serious cracks, b/c your W is married. However, for the WW, every thing is all about her feelings, and the more she gives way to her alter ego.....the more she is caught up in a fantasy of her own making. The attention from the OP is thrilling, b/c it appeals to her alter ego......which is not the person her family knows. The OP fantasy distracts her from the daily reality of life. IMHO, the fantasy state of mind could act as a blindfold in seeing the truth in OP. The WW's alter ego takes charge and partakes of the forbidden fruit, so to speak, and gets an immediate buzz. She goes back for more ego food, and OP lays it on thicker. The OP is giving ego strokes, and the WW is returning ego strokes. WW's alter ego tells her lies, as it grows into a monster, destroying M's, family, career, etc.
Honestly, I don't know why the WW will often affair down. I really don't think it is a conscious decision on her part. If you notice, her actions/attidude, etc., all seem unlike her normal self. Bottom line....it's all about how the OP makes the WW feel.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!