Just because something is natural, doesn't mean it's for the whole public world to see. If that were the case, one could walk around entirely naked, right? Since the entire body is natural, and I'm sure many people are quite proud of their entire bodies.

I personally don't want to see a strangers bare, pregnant belly. Nor do I wish to see a strangers butt crack, or thong underwear, or extreme clevage, etc.

Just because one is proud, unmodest, whatever doesn't mean the rest of us share those views.