1) Don't hit on your buddy's girlfriend.

2) Don't make crude comments about your friend's mother.

3) Don't ask out your buddy's sister without getting the "ok" from him first.

4) Don't hit on another man's girlfriend.

5) Don't "sneak a peek" at the urinal.

6) Don't ask out your friend's ex without getting permission first.

7) All of your friend's childhood and embarrassing stories are allowed when speaking to his girlfriend and family.

Note: There is an addendum to most of these guy codes, and that is that you can do numbers 2, 3, 4, and 6 if the woman in question is at least mildly attractive. Number one is for the most part untouchable, but depending on how hot she is you could always cite the "attractive woman" addendum, but your friendship is definitely over at that point.