If you think that watching porn is disrespectful, is it disrespectful to look at other women and finding them attractive?
It's not cheating. Although, you would hope the partners are open and honest. I think it is in the right that the wife should be open and bring the subject up though. The lack of being open is what likely led to the lack of sex in their later years. With porn, you really aren't receiving reciprocation, at least compared to people you might find on the internet. That to me is pushing the line.