I think the term "public" is being used two different ways in this thread:
I've had sex in bathhouses, bushes behind gay/nude beaches, and at a gay sex party. While this was with or in view of strangers, they were all willing participants (or voyeurs), and nobody's rights were violated. I'm not sure I would call this "public," but the presence of (willing) strangers has an appeal.
I've also had/seen sex in the shower rooms of gyms; this would probably be considered "public" and thus inappropriate, but on a couple of occasions it was difficult to resist. (Once at university I was seen by another student, and I stopped immediately...a few years later at a YMCA I was caught by a staff person, who joined in.)
Some guy having a wank on a street corner is definitely public and definitely inappropriate. It could easily be considered harassment as @flozen suggests above.