Sally Hemmings ring a bell? And remember, the standard set by Cain's accusers is the mere accusation. Also, how do we know none of the others had affairs. Sensibilities were quite different then.
Sure. What year did Martha Jefferson die? How old was T.J.? Did he ever remarry?
Jefferson does not appear to have ever cheated on his wife.
In fact, Sally was Martha's half-sister, read into that what you will.