For me it is quite simple. If I feel guilt then I am cheating.
There are so many levels, but I also have to agree that if it is a one on one interaction with another person, whether or not that other person is legitimate (and not some smelly old man sitting in his boxers and smoking a cigar) and emotions and feelings are exchanged, then it is cheating to me. Especially if it is in a true Dominant/submissive relationship. By this I mean that the sub, for instance, is interacting with the Dom on more than a purely "cyber sex/ ok I got off buh bye" kind of way. Once deeper emotions are exchanged other than a mutual quickie, then, imho, you are giving that other person something that belongs to your spouse or significant other and that is cheating them out of something you promised them. Make any sense? *grins* ~blizz~