The definition of cheating, to me, is quite simple - breaking the rules of the relationship that you have. If the SO knows and agrees to what happens and then that happens, then it's not cheating. If someone goes beyond the boundaries agreed-to, then it's cheating.
I guess for me and my own values it's not hard to know. The only time it gets difficult is when the boundaries are left undiscussed, and assumptions are made, based on some sort of societal norms. For this reason I am extremely careful to discuss as many possibilities with my partners before they become an issue, so that we can discuss them with level heads.
Redpepper, yes I read your story and no, it doesn't seem at all hard to know. His partner was not in a position to consent to changing the rules of the promised relationship. I don't think that a lack of ability to consent should constitute a legitimate reason to be able to throw whatever rules out of the window that don't suit at the time.
The whole purpose of a committed relationship is commitment - a set of promises are made with an expectation that those will be honoured by everyone involved. Too many people make promises without really thinking what they mean, and even whether they can really honour that commitment. This is, to me, what is what is primarily wrong with marriage today. "Defence of marriage", rather than being about who should marry whom, should be about enforcing the idea that a commitment made for life is exactly that, otherwise the commitments should be phrased differently.
If I make a monogamous commitment to someone (which I wouldn't, just sayin') and that person gets very, very sick, then my commitment to that relationship trumps any desire of mine to get laid.
This is (obviously!
) a subject I feel very passionately about - it's showing me that others have a lot more flexible idea of what ethics, integrity and honour mean. So it's showing me that I need to have a few more detailed discussions with my current partners and in future with any potential partners, to find out where they stand on this.