You might impress your girlfriend more by finding a non-violent responce to those who insult her. Standing up for your girlfriend doesn't mean that you have to hurt someone. Perhaps I'm misinterpreting what women want, but I've dated (and was married to one) women who were physicaly abused as kids. Men who are violent (towards anyone) when angry scare these women.
I, like you are a non-violent person. My wife, was physically abused by her ex. However, also like you I will step-up if need be. We've been married 19 years next June 16th. Someone famous once put it in a song. "Sometimes ya gotta fight when you're a man."