I prefer open relationships myself, so to me cheating is having romantic or sexual intimacy without getting the consent of everyone else you are romantically or sexually involved with currently. In this sense a few months ago I was 'cheated on' with a person I had expressly said was not cool with me. It ended up in a shouting match because I felt ignored and betrayed. I did try to salvage the relationship because I really loved my girlfriend, so cheating is clearly not an automatic deal breaker for me.
Sex, love, and partnership are sacred to me. But sacred things should be shared, not hoarded imho. Sure it feels nice to be someone's 'Mr./Ms. One and Only' until you realize you are far more likely their 'Mr./Ms. Right Now.' I fear that many people prefer Monogamy due to Insecurity Issues and justify it as 'being right' or 'what I am entitled to in a lover.'
Love is hard, and life is too short, imho, to discard relationships over sexual and romantic indiscretions by default. Ask yourself this question, "If I really loved her, would I leave her because she cheated? If the answer is yes without a thought...that is not love imho. Love is worth forgiving and forgetting imho.