Because it's polite and it limits the diffusion of bad smells. If you really want to see my dick all you have to do is go on chatroulette. No need to follow me to the toilet. Unless you're interested in more than just looking.
Point is not that you have an altruistic reason for wanting privacy for certain parts of your life (which I'm sure you do).
The point is that once the very act of wanting privacy starts becoming suspicious, we've moved away from a free society towards the totalitarian end of the spectrum.
