Need feedback

I have a few thoughts on this, and I'm happy to share.

First thought is in line with your thinking. I'm surprised that the therapist would suggest a group with optional nude time to a man with a history of infidelity with men.

My next thought was a couple questions: How long have you been married? What are your religious views/affiliations?

My other thoughts are about how you two are communicating. "I got triggered, went in a rant" - this sounds like it also triggered him, leading to - "my spouse got defensive and mocked me ONCE AGAIN when I was triggered" - and that led to your feeling that he had "minimized my reaction".
I wasn't there, so I don't know how the conversation went; but in my relationship, when one of us feels triggered, we excuse ourselves from the situation to process the feelings we are having in a way that the triggered part of the reaction doesn't poison the constructive part of the interaction. Imagine if your husband had heard (in a calm, collected tone), "Honey, I really appreciate your dedication to this recovery process and your continued efforts to find new ways to work on yourself. However, I'm concerned with the specific programs that your therapist suggested because of the inclusion of time spent in the nude with other men. I did a little research of my own and found 3 other programs that all look just as good as the program you were recommended. The advantage of these 3 is that they don't include any nude time with others. I don't want to tell you what to do, but I need you to understand that I would be incredibly uncomfortable with you going to an event that suggests or encourages nude time to be spent with other men."

 

There are a lot of forms of therapy, I suppose.

I can understand your concern and response;
would probably respond the similarly.

Is he in a program like SAA?
There are inner circle (no go things like porn)
and middle circle (slippery things)
that he should ask himself where the nudity fits.

My SO tries to be aware of these things
with me, too, and will try to help and be understanding.

Like you did in noting the nudity.

What does he say when things calm down?

 

Have you every heard of the marriage course? It might be worth checking out!