So OP, I don't mean to discredit your feelings, but in the past when people know you have autism/aspergers, do they treat you differently immediately? Because if they do, I don't think her parents will treat you any differently.
I think you have still never fully come to terms with the fact that you live with this and honestly thinking that you need to keep this a secret isn't a healthy way to live. So what if you have autism/aspergers. You're out living your best life. You have a girlfriend who wants to be with you despite it. Not everyone is going to treat you differently and I think your girlfriend is proof of that. It's okay to let your guard down once in a while. Celebrate that you are living with this and are still able to go out and enjoy life. Not everyone is so lucky.