I admit to sometimes posting those- that's how Ella the Cat Burglar got her start- but I always own up to that fact, and seek input to expand the thought into a more fully formed idea.
With this one, lots of questions could spark ideas, but the most challenging one is - how does it go from mob with nudity to actual sex?
will this be group sex?
or just a couple or threesome fucking in plain view?
how do others react?
do others suddenly start fucking too?
do the cops show up?
does it end up on the nightly news?
is it omniscient third party narration, or told from the first party POV of a participant or onlooker?
With the National Nude Day contest about to open, this could be the basis of an entry(though I already have mine).
A very short story, about the HR department at a large company having a collective stress related meltdown and flashing their colleagues. I don't see sex maybe masturbation.