Sorry if I'm a little late spotting your thread. Hope you are still around, reading & learning.
After reading your post, a couple thoughts came to me.......
First - because it hurts the most - the revelation about cheating etc. Trust is such a fragile thing. We like to think we can trust the people we love and who love us in any and all situations. Myself, I guess I don't believe that. I'm sure there are probably case where it's true but maybe I haven't personally come across one (or many) yet.
But here's the thing I think I'd want to zero in on regarding this.
Affairs and 'cheating' etc have ALWAYS happened. But ask ourselves why ? There's a variety of answers but complicit in them all is the underlying fact that we're raised in a culture where deep connections to more than one person (in a romantic or sexual manner) is a taboo. But it's an unreal and unrealistic taboo. Unsupportable for a very significant portion of the species.
But instead of having it out on the table for learning and discussion from the beginning, we put the blinders on and pretend it's really not that way. And then BOOM !
Hopefully in the future (some day) it will be otherwise. The emergence of poly into the mainstream offers some hope I think.
But in any case, I would only advise you to take this history into consideration. More than likely he proceeded the way he did because it was the only thing he knew ? Was it right ? Of course not. But we make all manner of bad decisions out of poor or lack of information. Give the devil his due.
But moving on............
The reasons he offered for pursuing his connection with GF are POTENTIALLY very real. The universe is funny. We sometimes to come into contact with people that seem to have something special that we need to understand. Often we aren't even looking. We just turn around and there they are. Poof !
This type of thing doesn't get a lot of press, maybe because there can be beautiful and special things surround them. And only the bad drama gets talked about. So accept the fact that this MAY be one of those ? But wait and see. Time teaches us a lot.
Whatever the case, you need to understand it changes NOTHING about the connection between you two !! Repeat 100 times
Unless of course one or both of you makes it change by over reacting to either the NRE/curiousity or jealousy etc.
I'd try to take a step back, dispense with any preconceived notions and take a look with an unbiased set of eyes. Hard I know after a breach of trust. But it's the only real path with heart. Start fresh, assume nothing, move forward from there based on what you see in FRONT of you - not what's behind.
There IS something special in here for you too - even though it may be clouded for the moment. Believe that - and watch for it. It may not show up in a day, or a week or maybe even a year. But it will sprout.
I know someone said they had not personally seen any success stories born of 'cheating', but I can vouch for several, my own included. And maybe the most important fact is that if you go through this together, it's almost certain that that option will never be an option for the future. It's like measles. Once you survive it, you never have to worry about it again - IF treated properly the first time. A blessing in disguise sometimes.
Good luck - keep us posted.