Look at the word you used. Cheated. You didn't say "when you find out someone slept with someone else", which implies at least the possibility of being honest. Cheating means dishonesty, and broken trust, by definition. Are you generally not bothered when people are dishonest? If the answer is no, then are you so far removed from understanding other people that you can't see why for most people the answer is yes?
see the review