I certainly feel that once there's a disintegration within a relationship, in walks a vulnerable state, hence the enormity of the feelings regarding the other woman. To me it's part fantasy, not that she isn't a very real person with very real interactions, and part escape from the pain/anguish/hurt/dissatisfaction/what-have-you that is caused by the void/instability of the marriage.

Not uncommon, what so ever. It does bring to light a crossroads.

Tough place to be in, for sure.