this season)
Apparently George and the neighbor Brenda have had a sort of emotional affair but never physically consummated it. Fans of The Big Bang Theory remember Sheldon telling Penny he developed the habit of knocking three times before entering a room because he "saw his father having relations with someone who wasn't his mother". IMO, Young Sheldon brilliantly (and admittedly) retconned this...in one episode, Mary and Sheldon returned from Germany where Sheldon was studying and Mary dressed up for George in the bedroom, donning a blond wig with braids and a traditional dirndl dress and speaking with a German accent. Sheldon walked in, saw a "blonde" woman speaking German, shut the door and "never spoke of it again".
If George had cheated on Mary at another point in the show's run, I'm not aware of it. The showrunners recently admitted they "made a mistake" making George so unlikeable on the Big Bang Theory, and with Sheldon's eidetic memory, it's going to be hard to credibly discount both his and Mary's comments about George's personality (I believe once on BBT, Mary said something about George being so dumb, he fought some kind of wildlife for some Twizzlers or something) and his infidelities.
