Think about it this way, if you moved out of your parent's house you'd have to pay rent, bills (gas, electric, water), council tax for your girlfriend (assuming she's not in any kind of education), car insurance, petrol, food etc etc. You could do a lot worse than paying your parents some rent, they'll probably charge you a lot less than any landlord would. Even though you may contribute to expenses now, remember that you're not actually paying for them in their entirety.
So in short, if you don't like it. You could always move out.