I used to know someone who, in situations like that, would sometimes actually lean over and politely say to a guy doing that "Excuse me, sir, but do you have uncommonly large testicles?" In a very prim and proper tone, so that it couldn't possibly me mistaken for some weird come-on. And when they inevitably got flustered and muttered something in the negative while trying not to make eye contact, she would say "Well then, do you think you could possibly bring yourself to close your legs so that someone else might sit down?"

This was generally when she was on her way to and from work, at a mainstream corporate job, so she looked very respectable, which probably helped with the WTF-ness of it.