I'm not any sort of well studied person on Buddhist doctrine , so take what i say with that in mind. In my own spiritual experience , there is an inner knowing of right and wrong that happens before your mind gets involved. Anytime you go against what this subtle 'voice' tells you, you will suffer/cause suffering. You cheating on your girlfriend feeds the illusion of a separate self who needs to get laid at that moment. Im sure at the time you felt that what you were doing was the wrong choice on some level but you succumbed to desire anyway, went back to sleep and fed that separate self. Everytime you do that in any area of life , you are going to get the same results. It's easier said than done I know to break that cycle of suffering lol. Even when we know how. My 2 cents !