how do you define cheat?
is deception part of it?
anybody who is not monogamous, after having taken wedding vows?

Reminds me of the Lenny Bruce bit about the man whose wife walks in on him whilst he is making love to a chicken. ("She meant nothing to me. I never really related to the chicken at the same level we do, and..... anyway it was you I was thinking of, all the time we were doing it")

what I mean is, its all very personal, all kinds of foul deeds keep a marriage together sometimes.