Actually it can be. Your girlfriend is in a bind. If she is “with” you, her parents will be upset. If she calls it quits entirely, you will be upset – and so will she. The way to shoot the middle is to be your very, very good friend but not to define the relationship as being boyfriend-girlfriend.
Right now she is dependent on parents who for some reason don’t approve of you. She can’t afford to do something dramatic like telling them off or packing her bags. If she loves her parents, she doesn’t want to damage her relationship with them either. She’s stuck. That puts the ball in your court. She can’t change things but you may be able to.
One way you can help is to do everything you can to improve your relationship with the parents. Of course, if they disapprove due to something unchangeable like race, this isn’t possible. But if they have some mistaken ideas about you, perhaps you can do a few things to change their minds. Set whatever anger you have about it aside. Ask your girlfriend to be as specific as she can about what exactly they find objectionable. Then make an honest effort to show them your best self. With time, you may just win them over. Your girlfriend will love you all the more for making the effort. And you will have made peace with people you might be related to someday.
I wish you well.