My girlfriend wants me to go to Thanksgiving at her parents this year, but I haven't even met them yet.
I don't envy you there. I have done the blind family meeting at a holiday dinner before. It's nerve racking. 
I feel a little nervous about it but my girlfriend doesn't understand.  She is starting to get irritated that I haven't given her my yes or no yet.  It is putting a strain on things.
Well, my advice is to give in now, because you probably will in the end anyway and avoid any extra arguing you two would do because of it.