I have my girlfriend pee outside because I didn't want to stop at a dirty gas station
Why would you not just find her a clean bathroom?
She doesn't make me feel in any way responsible for her problem
You shouldn't decide where people should pee. If they want to pee in a dirty bathroom, let them pee in the dirty bathroom.
That's not how my mind works.
You don't control your girlfriend to that point, it is unhealthy. If she wants to pee inside you should allow that. You can always wash your hands when you are done if you fear that it is too dirty.