There's always levels to things, a spectrum of acceptability, if you will. Public sex is a topic for another time and place - our social dynamics would need to thange dramatically to even permit for something way less radical such as more widely accepted public nudity (which is far more pertinent to this discussion, since it carries way less sexual baggage, even if it still weighs it down considerably). And yet, for some people, public nudity is an inherently sexual thing as well - even on a nudist beach. You don't see a whole lot of those in many places around the world where even the climate is perfectly conductive to such a venue. Somebody might take a peek over the fence and see something they were not "supposed" to see, after all... I guess there's a time an place for everything - including public nudity as a far more broad, not necessarily sexual notion - but if public nudity was progressively expanded to more and more public venues, the result might not even be a growing sexualization of the society that appears to be following it, but rather even a decrease of overt sexual behaviour, as social mores would still apply between naked people. Not every nude beach is also a public sex venue - and city streets would most definitely not become those even if nudism became commonplace.