I think it is about everyone's self image... or ego.
For some reason we have been trained to cover up our own selves and therefore need others to be.
However we must know that it is not every culture that is like this, as example Finland or Sweden, it is not rejected that men and women are in the same sauna or water naked together. This also reflects in advertisement... walk down a French Street and ads have nipples or bums exposed.
But the more "Puritanical" "Religious" countries like the US or UK don't like it.