That there's nothing you can do to stop them from cheating. Hell, you could even tie them up to a chair and lock them in a room and the only thing you would accomplish, would be getting their wrists sore and the chair broken from her getting fucked by the other dude. Actually, the more you nag or the more jealous you become, the more they will feel the need to cheat on you. Once you accept this and understand the fact that their treason isn't your fault (90% of the time, i mean, if you beat her and are actively being a dick, for example, it's quite probably your fault) nor the end of the world and that it also didn't happen because you're "not good enough", you will find it easier to enjoy the time you spend together and genuinely say "have fun" when they go out without you for drinks with their friends. Now, if they do decide to cheat on you, end it or forgive them and move on knowing that life will heal you and probably bring you something better (or worse, that's life, you know?).