As long as you get the docs, ID shots, etc. for the models, it's not really a problem.
In most countires public nudity isn't illegal, sexual acts in public are, but that's the problem when shooting it, not when buying the content.
Flashing converts nicely, there is a very successful public nudity site, german as well I think,
http://www.nudeinpublic.tv There aren't many sites of this type around, most are more beach voyeur style, or springbreak/mardi gras flashing.
Strangers are ok, if they are in a crowd or just walking by noone will care, but if you show a guys face closeup for 15 seconds, it could become a problem.
What you absolutely have to avoid is showing any brand names, no mc donalds in the background, no coca cola sunshade or stuff like this. Same goes for any music from a radio playing in a restaurant.