I guess I did expect some dismayed responses to the fact that he is cheating on his girlfriend. I have no intentions of defending his actions here - or anywhere. There simply IS no defense for something like that. But I wound like to clarify and reiterate a few things about it.
First off, I already know it is wrong. I cannot deny it. It doesn't do anyone any good to pretend otherwise. Ugly truths are always preferable to pretty lies. How can I know this and still continue to participate? Well, I could explain how it fits within my ethics, but imagine it would come off here as justification and rationalization. I already risk sounding defensive as it is.
Also know that if he showed any effort or desire to work on his relationship with her, or become honest with her, or make any improvement on that end whatsoever, I wouldn't hesitate to step back and support that. Hell, if I even just suspected that if I stopped fooling around with him, he would no longer cheat, then I would stop. Not that having that stance makes me a good person by any means, or lessons my guilt in the situation. It's only worth mentioning to help identify the options.
Another point I don't think I was clear on: I am not interested in furthering my relationship with him if he continues to date her. I don't see how there is any hope of him being fully honest with her, or that their relationship could ever handle a polyamorous situation. Not to mention that we have been fooling around for well over a year, the chances of her ever being comfortable or trusting this are understandably impossible. In my eyes, their relationship is already so poisoned and dysfunctional that I would never attempt to join it as it is. But more than the low likelihood of success - I obviously don't respect or care for his girlfriend. Pretending to do so would be false.
Fucking around is fucking around, and to me at least, that's every different from building an emotional involvement/commitment. I simply will not progress an emotional involvement in the situation as it is now, nor if he wishes to stay with her and be honest.
Although it seems their relationship is doomed, I do not wish to pressure him to break it up. I'd much prefer to support his efforts in whatever direction they point. If they separate, it must be for their own reasons. I know I've been in a relationship before that I knew should end and stayed too long, but there are important things to learn and I understand you cannot leave until you are ready or are done with it on your own terms. He has to figure out for himself what is going to happen between him and his girlfriend, not just respond to some ultimatum or pressure from me.
GS, you gave great advice. It is true: I mostly wish to let him know of the "potential". As SeventhCrow pointed out this IS likely to end in heartache, and there are many signs pointing to him not being interested in more. But there are also other signs pointing the other direction which I didn't describe, and I do trust my husband's judgment of the situation. I am more pessimistic than my husband about it, though, and am more likely to focus on the points SeventhCrow highlighted.
Despite all that, I still feel that he should at least know about the potential and my feelings for him. If I told him, I would stay away until he figures out what he wants to do about it. Problem is, I don't know how to approach him about this without scaring him off, coming off as pushy, or how to avoid misunderstandings. I already know that the chances of moving this out of the realm of deceit into something that can be built in a healthy way is slim, almost impossible. But I don't think I could live with my choice if I didn't take the chance because I was too scared. Most importantly, I don't want any missteps or accidents made now to hinder that potential future, no matter how unlikely it is.