Look at JFK. When you have money and “yes men” around you, you can do anything. But I think Paul had to be more normal than the rest because he came from a more stable family.
But remember JFK - he used to hide in closets in the WH with his women. And there was no way the Beatles had anywhere near a normal existence.
Paul and his family lived on a farm out in the country in Scotland. The White House has a lot of hidden spaces and a lot of rooms and a lot of people coming and going. JFK had more opportunities. I guess Paul could have cheated during the day, but if they never spent a night apart, then it hardly seems likely that he had the inclination to do so.