We cannot do everything at once, but we can do something at once. ~Calvin Coolidge
Trust only movement. Life happens at the level of events, not of words. Trust movement. ~Alfred Adler
Ironically, making a statement with words is the least effective method. ~Grey Livingston
I don't really have any good input to add but I did share a few quotes that I like...I am not sure the solution to this impasse. You are stating that in your case you WANT/NEED the details, the information.
He is telling you he knows better than you, that you really don't want to know, is that what I am hearing?
I can see if you wanted when/where/how often/what position-type details. Those probably would do nothing to help heal and just leave you with images in your head that you don't need.
But wanting to know what he felt he was missing, why he reached outside the M to find a way to meet his needs, etc., that is the kind of thing I would want to know, too. And knowing that he has a plan in place so that next time he finds himself feeling the way he felt then (should there be a next time), he doesn't turn away from your marriage, I would want that information as well.