If you're open about it ahead of time, that's not cheating. Cheating is when you go behind your partner's back with someone outside of the established relationship. Relationships are built on trust and establishing boundaries. Cheating (as the name indicates) breaks both of these. It's completely different from an open relationship due to one missing and _very_ important component: consent. If your partner is okay with it, have all the (safe) sex you want. But going behind a loyal partner's back and breaking their trust is of course going to hurt them. Even if they would have been okay with an open relationship, you not asking beforehand will have them wondering why you hid it from them, if they did something wrong, if they're not good enough for you, if you ever loved them at all, and what _else_ you might be doing behind their back. Your betrayal will have destroyed their trust in you, and rebuilding the relationship will be an uphill battle if it's even possible at all.