I'm going to go out with my friend's girlfriend
Why are you dating your friend's girlfriend?
Because my friend can be a real bastard to me.
Even if your friend is being inconsiderate, you should be loyal.  He probably never meant any real harm to you, but if you break up his relationship, you will be badly overreacting.  Besides, women should not be treated like possessions to be stolen.
It won't be stealing if she comes onto me!
And will you be proud of that? You are still doing something wrong.