What is going on with your husband is not polyamory nor do I see (from my standpoint) tendencies of such. Polyamory, first off, is built in trust, honesty, and communication - little of which I see in your husband actions. True he was honest with you eventually about this other woman but his actions dictate deceit and dishonesty.
Speaking to your actions in the future, it is purely up to you as to whether or not you allow this to go on and try to salvage the decades that you each have invested i your relationship.
His actions resonate with me as extremely selfish and without regard for your feelings. It takes a tremendous amount of self confidence and confidence in your relationship with your husband to shift from a monogamous relationship to an open one- you have to ask yourself if it is in you.... Do you really trust that he is committed to you? Or is he committed more to his own needs/ wants?
I truly wish that I had a miracle solution for you- it is very sad to hear of anyone who is faced with such betrayal and deceit but alas I do not.
I can only suggest that you talk with him and discuss your future together- search within yourself and find what you need to be happy.
I hope this helps and I hope that you find a solution to your dilemma that suits you both and keeps you happy.