It is exactly that simple. The only case in which is would not be is perhaps if we’re talking about an abusive relationship, but that itself can hardly be consensual and is a fringe case entirely on its own.
You can’t blame your immoral behavior on your emotions, alcohol, or whatever. If you don’t like your partner and no longer want to consciously be with them, then don’t be. Breaking the agreement you two decided upon is wrong.
If you’re not mature enough to handle a relationship and the mutual respect and loyalty that goes along with it, then you shouldn’t be entering one. There’s no sob story to be heard about how you couldn’t handle it and had to violate your partner’s trust.