By most of your definitions, it seems like you regard anything one does that one doesn't want one's significant other to find out about, it's cheating. Last week I spent a day watching television and didn't want my wife to know this. By your definition, this is cheating. I don't regard it so.
It seemed clear in the context of the discussion that we were talking about activities that you might do or share with a partner other than your spouse. I guess everyone else who has posted here thought that that went without saying.
"Cheating," in that sense, is a betrayal of your partner's trust, but it doesn't follow from that that all betrayals of your partner's trust are "cheating" in that sense. And, too, there are different degrees all across the spectrum. The severity of the betrayal can be roughly measured by how hurt your partner will be to discover it, and how hard you'd try to cover it up. I'd be a bit surprised to find your wife preparing to divorce you because she caught you sneaking an afternoon of television. If you didn't want her to know about that because you know that she didn't want you to do it, then it is still a betrayal of trust, but a relatively minor one. On the other hand if you just felt like you'd wasted the afternoon and were too embarrassed to tell her, then it's probably not a betrayal at all.
I think most people participating on this thread aren't looking for loopholes or excuses, though, so the sort of thing you're talking about never even occurred to them as a "betrayal of trust" in the sense we've been discussing. There's probably not much point in pursuing this distraction any further, then.
I seem to have a different definition of trust than most. To me trust isn't an obligation to full disclosure. It's a commitment of continued support toward one's partner. It's the assurance that the other will be there for you and support you, and won't abandon you for somebody else.
That's part of it. It's bigger than that, though. Trust means a certain sort of confidence in another person - a faith that they will not do things that they know would hurt you if you learned of it. It means conveying certain expectations to them and knowing that they'll respect those expectations. The specifics of those expectations vary with each relationship, but most people know what their partners expect of them. If they find themselves doing something that they know their partner would view as a betrayal, then that's a violation of their partner's trust.
It's not a matter of "full disclosure." It's not doing something in the first place if you know full well that disclosing it would hurt your partner. In the simplest possible terms: Your partner has certain things that he or she trusts you not to do. You probably know what those things are. If you do them anyway, then you're betraying your partner's trust.
