Since the Coopers don't share their feelings and obviously George didn't talk to Sheldon about the situation when he walked in on George and the "other" woman and since it happened in the Coopers home.... My personal backstory was that it wasn't another woman but it was Mary with a wig on trying to spice up their love life.
And Mary's negativity about George is because she's still mad at him for dying young and leaving them.
After all, when he came to Sheldon's wedding, Georgie told Sheldon he didn't really remember the way it really was when George died. Georgie said Mary went to pieces and it was Georgie who held them all together.
