My own, perhaps idioyncratic libertarian view, is that because most public spaces (streets, sidewalks, etc.) should actually be private (owned, e.g., by homeowners' association), but are nevertheless public, the government can and should try to mimic the rules for public spaces that private owners would impose. There is no doubt that the vast majority of private owners would prohibit nudity, sex, and urination in public view. Those are easy issues, because the First Amendment is not implicated, but I think the same considerations apply even in situations where the First Amendment is implicated, and I'd argue against allowing picketing in front of someone's home, or protesting funerals, or public obscenity ("fuck the draft") [contrary to some commenters, I don't feel any obligation to defer to the Supreme Court regarding what I define as "obscene"]. I'd make some exceptions for the "public square" when core freedom of speech and association is at issue, but not many. Protecting people in "public" areas from things that would never be permitted on private property strikes me as going to the core of the states' police powers.

UPDATE: Many commenters seem astonished by the idea of private streets. My parents have a house in a private, gated community, where the streets are indeed privately owned. Access to the community is for owners and their guests. The community functions quite well, as far as I can tell, and, among other potential advantages, there is virtually no crime. This may not be everyone's cup of tea, but there is nothing either radical or impractical about streets being privately owned.