Because human psychology is more complex than the bland and banal morals you acquire from Hollywood movies. You can love one person with your whole heart and lust for another with a searing desire. You can do it because you want to prove something to yourself, because of some kinky thing you never get from your partner, because you're not sure what you want, because your entire body demands it, cries for it, screams for it.

She might have wronged you and it pushed you over the edge. Maybe you just got seduced? Maybe you were so drunk you thought you won't turn down your buddy who proposes you approach two girls sitting in the corner.

Do I really have to keep going here? Just how one-dimensional do you think humans are?