Thanks for the clarifications! I'm glad you've moved away from the "path of least resistance" in your first posting, where you seemed to be titillated by the "gorgeous friendship of a man who is married" to more awareness of what you feel is acceptable. Kudos for treading carefully until you have more information, and I'm glad the date with theatre-guy went well. You sound like you'd be a lot of fun to hang out with, so you'd have a lot to offer in a relationship!
SSM,
I don't know many people who would consider it an "affair" if both spouses agree to part ways before engaging new partners. That is responsible and mature. The kind of affairs described in almost every situation in Newcomers, MLC, Separated, Divorced, etc, here, involve one partner being blindsided by the discovery that their spouse has developed an EA or PA on the side, and yet has blamed them for all the problems in the marriage to assuage their guilt (and yes, has lied, lied, lied--the emphasis is for the sheer volume, and for the utter shock their partners feel at this). Their raw agony at discovering their "best friend" has betrayed them by sharing confidences, and/or sex, with another, is the worst experience of their lives. It doesn't hurt any less to be what you dismiss as a "stereotype."
