1) Condoms doing no matter who, boy or girl (yes, that includes your girlfriend). Dick-eating sex bugs aren't something you would want ever. Keep some in your bag or wallet or whatever.
2) Girlfriend's consent is required. Yes means yes. No means no. Not many things suck more than dealing with a crying pissed off woman who might never trust you again and eventually ditch you. So think carefully on that one if you care for her.
3) People have soul, so common sense of respect do not stay at the door when you get in bed with a person. Even if it's just to get off for one night only. If the person is actually interested in you beyond sex, avoid him/her because it's not the right one. No one really likes a heart-breaking bastard.
4) If you have a girlfriend and he doesn't, why feel insecure? If I were you, I would have something to brag to counter his own bragging and get all holier-than-thou on him.
5) Have a huge dick.