I talked to a girlfriend about this and she says it's because he is realising that the weird arrangement we've had going for the last two years (where he got to have his family, his home and his single life) just won't work where there is a third party involved. The limbo/equilibrium is broken and it is making him uncomfortable. Plus, my apparent 'ok'ness with it all probably doesn't help. But honestly, I'm not sure how else I'm supposed to be.
I think your girlfriend is right. He was probably, in some way, in denial about the state of your marriage, what it would be like to have it end, what he has now that he quite likes if you were to formalise your arrangement. And how he's been honest with you about his relationship, and you have reacted as an ex-wife would, with dignity and disinterest - well, the reality of how things are is hitting home. I don't think you can or should be anything other than what you are right now.
