Why would someone say it's "not cheating" if the couple isn't engaged or married?

If a couple is in a committed, long-term relationship, and they have both agreed to be exclusive, isn't it cheating and betrayal for the woman or guy to have with someone else? I don't understand the logic or a person saying that it is NOT cheating! I've heard that repeatedly lately...That it is not cheating unless there is the ultimate committment of marriage. Can someone explain to me how that can be true..Seems to me that betrayal is betrayal...And it shows the character of the person to break that trust/loyalty.