Emotional rather than physical obv!
I know someone (not me, I promise!) who reconnected with an ex on FB and began putting all her emotional energy into that. She said that she didnt want an affair with him or to wreck her marriage but that they had just clicked. I think, and I am sure her DH would agree, that what she is doing is already an affair but she doesnt see it. She showed me the messages to prove there is nothing in it and there are some that arent......inappropriate exactly but more in-jokes and "do you remember when...." that allude to their previous sexual relationship. I would feel that was very wrong if I were her OH, but as the wife of a man who cheated, my radar may be skewed.
I said that anything you cant share with your OH is deceitful but she just says that it is none of his business. She loves her DH but I think she is doing the Shirley Glass thing of opening a window to this other guy whilst building a wall between her and her husband.
Or is she just very good at cheating if even I am not sure she is doing it?