I'm far from a prude, but from my perspective, public sex -- in venues like store bathrooms, gyms, highway rest stops, and urban parks to name a few -- is inappropriate.
Often enough, the participants are caught by surprise, while others find a thrill in the occasional exposure.
In either scenario, when public sex acts subject other people to an unexpected and unwanted experience, it's sexual harassment. These third parties who stumble in on such antics can be parents with small children, for example.
Watch public sex fantasies as porn, if it suits you. But in today's real world -- with the Internet, clubs and other ways to hook up -- it's time to move on.