In the ensuing 50 years has he given you any reason to think that this was a pattern or a one time thing that happened in the 70’s?
Let’s say he made a stupid mistake once 50 years ago and has been a good partner since then. How will obsessing over this help you? Let’s say that you learn the truth – what will you do? What line has to be crossed for you to walk away now when you’re in your 70’s?
Let this go. If the truth finds you… deal with it then.