Here's a philosophical question for you:
If a man strips in the woods, and nobody is around to see it, is it public indecency?
Classically, this question is moot, since if nobody sees it, then there's nobody to report it. But in this day and age of photographic evidence (oops!), when it's a trivial matter to hide cameras in trees (are we paranoid yet?), it makes you wonder.
I don't believe public "exposure" is that harmful in the first place, and I think some people have a valid desire to be nude among nature - and they don't always have ready access to the proper nudist facilities to do so.
So my approach has always been, if a person takes reasonable precautions to avoid being caught - thus proving that they're not simply pests trying to shock unsuspecting passersby - then it should be considered "no harm no foul". As outliers in the social construct, they deserve at least a little bit of leeway to pursue happiness in their own quirky but altogether harmless way.
And I really believe that. But mainstream opinion is not so tolerant, and this causes me a lot of anxiety - I feel like a fugitive criminal when I engage in these kinds of behaviors. People commit minor crimes on a daily basis (e.g., littering, jaywalking, speeding) and it's no big deal, even if you get caught. But those kinds of things don't have the same stigma as anything that involves nudity or - heaven forbid - sexual stimulation.