Find common ground.
At the end of the day, meeting her parents is not unlike meeting anyone for the first time. It's about searching for things you have in common and respecting what you don't.
The easiest way to find common ground is to ask questions. Where did her dad go to school? What does her mom do on the weekends? Be interested in who they are and what they're about, and they'll be less averse to seeing your face again.