Personally....you did the right thing.
Women dont like douches that start fights. Women in general hate conflict. You intervened on the girl's behalf and stopped the guy, that right there is something not alot of people have the courage to do. You dont need to justify your actions by beating that guy to a pulp.
Really, your own male ego is hurt by this. The woman looked at that fight totally differently. She saw a guy that stepped in and stopped the harassment towards her.
It sucks that it happened bro, but it's not a DHV to beat a guy to a bloody mess. You did what you had to and was classy about it. Try not to throw first when there are women around IF you can help it. They'll think more highly of you than that other jerk.
If you were alone though, I'd be flaming the piss out of you for not swinging back
I think you did the right thing. Dont let it mess with you too much. You did what was the right thing to do, you dont need to walk away from all of those fights a winner. Be happy you did something. Imagine if you just sat there and let him do what he did. You'd feel like total shit, wouldn't you?