Thank you both for replying!
It could be difficult to understand why I would stay with him and want to continue to work and build when he has this history. I wasn't 100% sure what to include initially but I can see now that perhaps it's important to highlight a few other things.
I deeply love this person and he is my only family (I'm strongly estranged from my family of origin- they are a very toxic system). I emotionally distanced myself from him after the initial betrayal 4 years ago because I have self-respect and didn't want to put myself in the position of being vulnerable to someone I couldn't trust. Yet, we have a delicate family situation (raising a special needs teenager without much support) and I wanted to act in the best interest of our son. I'm pretty clear that this was a good decision- it would have been incredibly hard on our son on many levels if we'd split households. So, we've been sleeping separately and co-habitating as co-parents with this veto option in place as a means of protecting who has access to our son and our home.
Then, last fall, my partner realized that he has some fairly serious issues and finally sought help and was diagnosed with Borderline Personality Disorder. Luckily, one of the best treatment centers that specializes in this is in a nearby city and he has been working very hard with a team of therapists for almost a year. The change has been amazing! Yet, it doesn't happen overnight and I believe that his lapse in judgement concerning this young woman (and deceit that covered it up) was an example of a huge step backward in reaction to feeling panicked about having made so much progress.
In short- I have a lot of reasons to believe that trust can be rebuilt and while it's hard to articulate these reasons, I've been really clear with myself about my own needs and boundaries. Now, after this situation has blown up, we are romantically and sexually together again and this past week has been almost constant communication, working things out as they arise, and the depth of our intimacy is something I wouldn't have believed possible.
It will probably be at least a couple of years before we will be ready to welcome someone new but it's a really important piece for me that, as we are laying the foundation of this new relationship, the expectation of a third person is built in.
It's also worth mentioning that another factor that has changed this year is that I've transitioned genderwise. Before, I was trying to have a relationship with him as a gender that I am not and this really prevented a lot of intimacy and authentic connection. Now, as myself, I'm able to share myself with him in a completely new way and he responds to me differently than when I was presenting as a female.
SO... yeah, lots of factors and it's a lovely jumble of human experiences.
My question remains about how does a couple go about dating/ finding a mutual third? It's going to be a while but I want to be thinking about it now.