I'm going to stick up for my other girl-friends while I have a gf
Why bring up your other girlfriends while you have a girlfriend?
I will do it because I like to make them happy
It's great to stand up for your friend because they will remember it. When you are in trouble one day, they will probably be inclined to help you instead of ignoring you.
That's a good assumption.
Standing up for your friends is great regardless. Just be careful to not make your actual girlfriend jealous. Your focus should primarily be on the person you are currently in a relationship with.