@outerspacepotato you are right, of course. And I've been having a think about everything because there have been some posters who just wouldn't do what I am prepared to do for my marriage. I get it.
It is so hard to explain, but, as @Allthegoodonesareg0ne so helpfully wrote:
I knew that he was doing a selfish and hurtful thing but I also knew who he was really. I knew he was going through something and had made some terribly hurtful choices but I felt strongly that I knew this wasn't what defines him.
I know that I do not have to put up with anything that makes me uncomfortable, I can walk away, stop it and get divorced and all of that. But, rightly or wrongly, I still see that as a lose/lose at the moment. It will get to the point where I wont give a shit, I know that. The time hasn't come yet. And honestly, maybe I am being unfair on him, because I haven't told him that will come. He's not stupid though.
He knows what he has done, how it looked. I see him in pain, wanting the future to be okay for us but also wanting his hobby friend in his life, albeit under a totally different set of 'rules'.
He has said that he won't DO the hobby with her anymore, but instead because every now and again they'll inadvertently be in the same place at the same time, they can enjoy the hobby together. They're 'just mates, after all' (HA!)...
He says he had stopped texting her before she went of the radar, and he thinks that her not being in touch with him fixes everything. It proves it was only platonic, and not a big deal (HA!). He is worried that he'll see her at the hobby, and he will find it hard not to say hello or acknowledge her because he's not like that. I find that easy to believe, knowing him as I do.
I know I can't do that.
I know that he shouldn't give a shit about her - only us. But he is going through something, and I want it to work it's self out, together. It's the only way I think we can move forward.