It depends on the person's belief systems. Several people say it's only cheating if you get caught. Some others say it's cheating if you even have thoughts about being with another person. I would say that the common man falls in the middle and says that head is cheating. Anything that you wouldn't do in public with a buddy, boss, or family member is probably considered cheating.
But if a tree falls in the forest and there's no one there to hear it, does it make a noise?