I tried to reply earlier, but my internet service provider decided that it wasn't time for me to do so.
I think it's great that everyone has been trying to give you advice on what to tell this girl, but.....
I say
tell her how you really feel. If you really feel that it's stupid that Christians "can't" date non-Christians but you're doing it because you honor your parents, then tell her that. If you think that you don't care (I'm not being facetious or holier-than-thou; you have the right to decide) what Scripture or your parents say (or seem to say, as certainly the "be not unequally yoked" passage is open to at least
some interpretation), then decide that for yourself and decide whether this girl is more important to you (again, I'm not being facetious at all).
All of us can give you great theological arguments or advice from our own lives, but it's ultimately useless to you because dating is such a personal thing and your commitment to follow Scripture and your parents is also such a personal thing. You really need to think this thing out for yourself, decide what's important to you (still, not being facetious), and act accordingly. Whatever you decide, I say just tell the girl the truth (whether it's that you're personally convicted that dating a non-believer would not be in your best interest, whether you're only doing it for fear of disappointing your parents, or something else). You'll learn rather quickly as you get older that people value honesty over "white lies," especially in relationships.