I'm sorry for what you're going through.
I didn't know while we spent many many nights and hours discussing this openly and agreeing that it made very good sense that my spouse was already practicing.
Unfortunately, your husband was NOT practicing what you were talking about. If you didn't know he had another lover/partner, then he was cheating.
If his other partner senses your discomfort and withdraws from your husband because of it, in my opinion that's entirely his doing and you shouldn't let him shift the blame for that onto you.
Have you asked your husband whether his other partner knows you were ignorant of their relationship until recently? Technically, she might view what he's done with her as cheating, too, and she might be as upset about it as you are. Many would agree she has every right to be (though opinions differ and the ethical issues of enabling cheating are some of the more hotly debated issues amongst respectable poly-folk.)
I think it's honorable and the right thing to do to try to save your relationship. However, if you have the slightest concern your 33 year union might end because of this (willingly or unwillingly), I think you should consult with a lawyer sooner rather than later, to at least learn what rights you might be giving up by taking certain actions.