Hey sport, first off you're not marrying her parents. We need a lot more information before I can give you great advice. For example:
If you're under 25, who the f*** cares what the parents think. In fact, why even play that game, you're too young.
How long have you been dating this girl? The parents probably know you're just a phase and not really the "for ever" kind of dude. BTW, is your girl still living at home? Ouch, I can barely remember those days if that is so.
Give us some more DNA and then we can figure out how to guide you through the up-coming storm.