Like others have suggested, you can never state "I would NEVER do XYZ" until you are faced with the situation. Yes, you too might eat human flesh if actually finding yourself stranded on that lifeboat at sea...
Also as others have stated, I fervantly HOPE I would not betray trust or break promises (yes, a promise to be exclusive/monogamous is your word as much as a marriage vow is).
I think asuperwoman's idea is an interesting one--that the cheating is all about the cheater and doesn't really depend on the actions or personality of the partner they are cheating on. There seems to be truth in that.
However, having said all that, I know from my past behavior that I have a problem with guilt (carrying it basically for the rest of my life) and that I would be extra-hesitant to get into a situation that would trigger a lifetime of guilt.
But the single biggest deterrent to my cheating would be thinking of the super-human kindness and goodness of my partner (of 6 years and he's the only man I'd consider marrying one day down the road). The thought of harming an innocent who is not only himself loyal and faithful BUT who blissfully believes the best of me and has implicit trust in what he thinks is my own goodness...this would be a big red stop sign to me.
I agree with asuperwoman that we can't blame the cheating on the cheater's partner/spouse. But, to be totally honest with you, I do believe that I would have an easier time (conscience-wise) cheating on a man with several irritating or offensive character flaws (petty, secretive, purposely ornery, controlling, immature, etc.) or whom I've been fighting viciously with than with a man I know to have the heart of a newborn lamb.
It's sort of like how many meat-eaters don't think twice about consuming adult cow, sheep, or fish, but bite their lips over the thought of veal, baby vension (i.e. Bambi), or other cuddly critters we idealize.