I hate people like that...

I don't understand why anyone goes after a person who is in a relationship. Yes, it is wrong for the person in the relationship to cheat, but somebody else shouldn't knowingly go after a person in a relationship. There are plenty of other fish in the see. If your connection with that person is so strong, than the one in the relationship should be a big enough person to end their relationship. I just don't understand why someone would want to knowingly take part in hurting a person who probably has never done anything to them.