Some philosophers have practiced sex in public places. Diogenes the Cynic engaged in public sex, though only with himself.
His point was to confront his fellow Athenians with the truth of their animal nature. Most of us, however, prefer a life of self-consciousness and feel some pride in the fact that we are not merely dogs, from which the word Cynic derives.
I suspect rational consciousness, which is always self-consciousness is the reason this taboo is so nearly universal. There have always been exhibitionists, voyeurs, middle-class "swingers," romantics, and radicals who promote such "liberation" from social constraints.
But as far as I know, modern philosophy have not devoted much time to this particular problem. Possibly Zizek has something to say about it; since he has something to say about everything. If sex in public can be shown to be moral in a deontic sense, then the State should certainly make it mandatory.