Unless previous girlfriends or friends have told you you have a habit of excessive ogling your gal is probably a bit neurotic about this sort of thing, and that's not something you can really fix from your end.

That said you might want to double check to make sure you've got a decent poker face, I remember I had a friend complain to me of this once over lunch, and ask if I thought he did that, and it was everything I could do not to start laughing since most of our conversation had been carried out between him eyeballing every passing skirt and offering commentary like "Nice" and "Come to Daddy", yet he was totally convinced he didn't have a wandering eye... which was true, his eyes never wandered, his whole damn head would turn like he had a targeting computer locked on. So you may want to make sure first