Hello and thank you for your question,
Yes you have every reason to not be comfortable with his communications with his ex. He is not just being friendly or catching up. He is being flirty/intimate and also hiding important facts from her, such as he is married! He is also dismissing your concerns and minimizing the impact that his behavior is having on you and your ability to trust him.
The fact that he said you ruined the relationship between his co-worker is an odd way of looking at the situation. He could have shown more concern for your feelings rather than focusing on what it did to "their" relationship. Since he won't go to counseling, all you can do is set a firm boundary with him and not just let him off the hook. Tell him that you do not want him to contact women (in this manner) anymore. That his wording and manner is not appropriate for a married man, and telling her IMY is more than just remembering old times. That even if he doesn't see it that way, that you do, and it affects your trust in him and that you deserve more respect than that. That you would not behave this way in regards XXXXX XXXXX men and that If he continues, then let him know that you will be considering whether you want to stay in the marriage. Then go ahead and make an appt. with a therapist and let him know it's scheduled and he needs to show up.
Don't allow him to not feel any consequences for this breach of trust with you... Please click ACCEPT button for this answer. And feel free to continue the discussion even after clicking accept. Thank you.