How are you supposed to react when someone's ogling your hot girlfriend?

Tell her that you don't want to fight them to get them to stop. You could also tell her that it makes you feel good when they look at her because it tells you that you did good and have something good. But if they do start commenting and whistling, you still don't have to fight. Just confront them and tell them to stop. She should understand. And it'll be stupid if she doesn't. She needs to respect you and what you want.