Cheating is defined as an act of deception, fraud, trickery, imposture, or imposition. Cheating characteristically is employed to create an unfair advantage, usually in one's own interest, and often at the expense of others. Cheating implies the breaking of rules.
I hear people say that it is not cheating if your spouse is ok with it. But breaking marriage vows is cheating even if you agree. If you both agree to rob a bank that does not make it ok.
I think cheating is less harmful if you both agree as in open marriages - but harmful nonetheless.