luchog
Neo-Post-Retro-Revivalist
It's mostly esthetic. Most people don't look good naked.
Except that's demonstrably not true; otherwise there would be similar attitudes toward ugly people and spandex. No one who is not a serious athlete looks good in spandex.
As others have pointed out, it's ultimately about "decency" standards that are derived from religion, and at best are supported only because they've become customary.