This IMO is one of those occaisions where the rules aren't equal between the sexes and it's fine by me. A guy shouldn't do this, but if a girl wanted to give you a pinch on the @ss, be thankful. I would and think of it just as some harmless flirting. And her beauty shouldn't be a factor whether it's acceptable. It's not that you're going to sleep with her. I'd give her a wink and a smile.