Thank you GalaGirl and Leelee.
Leelee- As for her behavior being "wiggling out" so as not to feel responsible for leaving, I definitely think there is some of that there. And I appreciate you having to guts to tell me that. That said, I know she values our relationship and does NOT want it to be over. We've been through a hell of a lot together, and we do have a solid partnership of 8y. I basically think she is just lost in NRE, and wants to go do her heart's desire, hoping and counting on her ability to come back to me later. That's part of why I'm so worried about the future. This may be a possible way to deal with this in the current situation (where the other person isn't really "eligible" for anything more than a fling, given age differences and family situations), but it does not bode well for our ability to face the next non-monogamy challenge.
I do share with her my fear of the future, and my disappointment with her for enabling a cheater. I knew from the beginning this other person was married, and both me and my wife assumed (wrongly) that she was happily so. We did not want to jump into conclusions as to whether the other couple was monogamous, and after the sexual tension became obvious my wife went to meet her and clear things up about what boundaries were relevant for each. That's when they first kissed. The cheater then convinced my wife that her marriage is her own business, and should not worry her (not to mention worry me).
I found myself trying to be the voice of reason and moral, to someone who really doesn't want to hear any of it, going crazy with passion and wanting to break all the rules. And at some point I stopped fighting, fearing it will distance us from each other if I kept pushing on the "cheating is wrong" subject.
Not to mention the fact that whenever I address it I am being accused of taking on more responsibility than is truly mine to take, butting into other peoples' relationships (as I am not the one cheating or cheated with, I am not involved in her opinion) and/or hiding behind morals to ignore my own difficulties regarding the opening-up of our relationship.
I have trouble discussing this with my wife because I know she is disappointed with herself as well, and going through some serious self-doubts as a result of her behavior, and I am trying to give her time to sort through this. I sure hope I would have had the strength not to go through with it if I were in her place, but I also know people aren't perfect and don't always live up to their own moral standards. It is what it is, and I don't want to be all "high and mighty", as she seems to accuse me of being in this matter.
Your comments did help me realize this "cheating situation" may have more to do with my issues than I was willing to believe thus far. I should (and would) discuss that with my wife. Maybe if the next time this happens the situation would be more of a true poly-ship, things will be easier. Thanks for the input!