When is it illegal to photograph someone in public? . . . even when showing too much skin?

I was outraged when I first saw the headline:  "Upskirt Photos Don't Violate a Woman's Privacy, Rules D. C,. Judge."

My assumption was that this was another clueless, probably older, male judge who lacked sensitivity to women's vulnerability to Peeping Toms and to creeps using mirrors to take photos up under their skirts.

Not so, not at all.   Turns out the judge is a woman, and the situation was quite different.    The photographer was a man, that's true.   But he did nothing out of the ordinary to get his pictures (like using a mirror).  

He simply photographed some women sitting on the steps of the Lincoln Memorial.   They were wearing dresses or skirts, and apparently they were sitting indiscreetly so that their crotches and butts were partially exposed.

He did not photograph anything that was not plainly open for anyone to see who happened to be in that spot.

Although the judge said she found the photographer's actions "repellant and disturbing," she was not convinced that what he did warranted his being arrested.  She dismissed the charges.

Categorically, is there any difference in photographing a crowd scene at the President's inauguration and a crowd sitting on the steps of the Lincoln Memorial?    If women do not want to have their nether regions exposed in a published photograph, they should not sit in a way that makes this possible -- so goes one argument.

What about at a nude beach?    Or a locker room?   Or a celebrity at a cozy restaurant with someone other than his/her spouse?

You could say that it should be illegal to photograph someone in public without their permission.   But photographs can easily be taken without a person being aware they're being caught on film.    With a telescopic lens, or a smart phone camera in almost every hand in public, how could you possibly know you were being photographed in order to object? 

The responsibility could be put on the publisher for print media.  But then there's the internet and all the other social media, where anyone can put up anything, without editorial oversight.  

The article on Huffington Post about this ended with the statement:  "Just because women's bodies are in public, doesn't mean they are public property."

That's a sensible statement.   But is it true?   Of course, not public property to abuse;  but, by appearing, are you not making yourself available for viewing?  Admiring?   Does it then become wrong if the viewer ogles lustfully?   Suppose that admiration is expressed with a whistle . . . or a prosposition?    When does is become wrong?   Is it wrong just to photograph . . . or only if the picture is published or even shared on the internet?  And if you say any of that is wrong, how on earth do you regulate it? 

I don't know.

Ralph