It's difficult to put my rather.. simplified opinion of the situation, directly after Myriads always insightful explanation of a situation, but I'll try.
To me: Cheating is something you do without your partner's knowledge, of an act that you know turns you on sexually.. or.. that your partner does with someone else.. without your knowledge., that they know turns you on sexually.
For example: Those who know me, know that I have a preference for womens feet and tickling women. So,,.,. when I do have a partner.. it would turn me on sexually to either play with their feet, tickle them, or have them tickle me.
If, without my partner's knowledge.. I made a date to play with another woman's feet, tickle another woman, or have another woman besides my partner tickle me, to me, that would represent cheating.
Conversely, if.. without my knowledge.. my partner told me "I let another guy tickle me, or play with my feet". That would constitute cheating, and to me, would be just cause to examine the relationship, and perhaps, end it for cheating, just as I would understand if the woman ended the relationship with me for such actions. I don't mean, of course, if my gf received a playful tickle from a long time friend, or family member, that isn't cheating, but the other explanation I gave of it is.