Yes, @rockvillemom, I did give that some consideration, and, thus, factored Diane’s sense of embarrassment in. But, having watched the show (I thought without interruption), I was wondering if it was a possible dual use of the word, one designed to lead the viewer to assume the more unscrupulous definition of affair, where there is also the less seedy use of it which may be applied.
But, okay, given your comment, I’ll go with writers cramming everything in to finish off story lines, and go with unfaithful to Diane. (I had such a different view of him for so long. Sadness.)