I think at this point, I would write a note of some sort. The problem with these situations is that you end up going in circles and arguing the same point. A note allows you to get your point down concisely without him trying to twist your words and make you sound unreasonable
It’s a well worn and pathetic path that your H is currently on One trodden by many men who have found themselves having a midlife crush. It’s selfish and they become so torn between wanting to pursue the crush whilst maintaining their “decent man” persona that they literally start lying to themselves and turning on their partners. In other words, it turns them into Utterly self-absorbed pricks.
I would keep the note brief
Dear X
To be clear, Your friendship with Y has overstepped the boundary of what is reasonable in a committed marriage. I know this, and for all your bluster, I believe you know it too.
In the many years that we have been married, I don’t think I’ve proved myself to be particularly jealous or possessive and yet here we are. I have told you that the relationship makes me uncomfortable, and rather than taking a step back, You have delivered an ultimatum of ‘ ‘Accept it, or the marriage is over’. Unfortunately, that ultimatum speaks louder than perhaps any of your actions that have led to this
If that is truly how you feel, Then neither you nor I can kid ourselves that this woman is just your mate. You have a crush on her. Or something more. It’s certainly enough for you to put your marriage in jeopardy
It’s heartbreaking that it has come to this, but I am not going to play ‘ cool wife’ whilst you make a fool of me
Let me know how we should go forward.
Wife