Perhaps this woman was a kind of "therapist" for your husband, did you ever consider that? Men can have a harder time emotionally and they can get very lonely. Perhaps that was something your husband was experiencing and then he met this older woman who had/has a more positive outlook on life and he found her a very pleasant person to be around and part of that could also include that because she was also older and married, she was safer in that the relationship actually was really "just" a friendship relationship.

I think that you want certain things from your husband that he genuinely doesn't "know" how to provide for you. A friendship like you describe can become an actual vehicle for learning and in this learning can help him see relationships in a new more positive light overall. Perhaps through this woman he was learning how to be more of a free spirit instead of feeling "alone" and having to know how to be a perfect husband or anything perfect for that matter. And that is part of these butt jokes in that one can learn to laugh and another's big butt instead of embracing perfection and criticism. When someone's smile imprints in one's mind, often it's not so much the smile, it's all the things behind the smile that revolved around looking at life in a much more lighthearted way that can be so freeing. I bet she makes him laugh and laughter is ALWAYS good medicine.

Do you ever just "laugh" together? Or is everything just serious. Did you ever think about getting to know this woman yourself? You may actually like her yourself, but that will not happen if you only see her as a threat or some kind of competition.