I think you already know the answer to that question.
How would you react if the table was turned? Before I do anything that I may have a question about, I always ask myself..."how would I feel if he did this"? If I know that I would want to break his neck, I don't do it. If I HONESTLY would not even get upset if he did it to me then I don't see the problem. This idea may or may not work for you, it depends on how you agree/disagree on certain subjects.
You are lusting and getting pleasure from a woman other than yours...... yes it is cheating no matter how you look at it. "cheating" does not require penetration you know.