Starsky, I did confront my W about the EA. I asked when it started, at what point she realized she had crossed a boundary between what is appropriate for a M woman, and if she felt guilt. She explained that this OM is someone she had known as a kid, briefly dated as a teen, then lost track of for 20 years. She "reconnected" with him about a year ago on Facebook as he was going through a marriage break-up. He is in the divorce process now and according to my W "his W is a psycho". I'm fairly sure my W and OM have had discussions about a potential future together, but they are waiting for the dust to settle on their current sitches before they move forward.
A good description of the problem, bboom. Now where's the BOUNDARY (including consequences) in there?