I rarely use a restroom in an airplane but if the trip were long enough I certainly would. And it wouldn't occur to me to wonder if it was a man or a woman who last used it...

Now back to the original thought- in my opinion if a person is quick witted enough to fire back an appropriate reply, as Dave Polshaw did, it's all to the good. Better a laugh from her companions at her expense than to be uncomfortably aware that she was rude. Because you em>know that, whether or not they spoke up, they were are aware of it and thought less of her for it.