Why is cheating so bad?

Before you answer, I think it's horrible. 
BUT, I want to know what you think MAKES it so horrible. 
If it's just the trust factor, then why don't we just say "ah go ahead and sleep with anyone as long as you tell me first."
If it's the fact that your partner is sharing his/her body with another person that isn't you, then why wouldn't you get upset that they had done so with another person BEFORE dating you? Like why doesn't THAT knowledge hurt for some of you, but for some reason, if your partner has sex with someone else at the SAME TIME as dating you, that does hurt?
Trying to get to the bottom of what it is.