Its been touched on before, but how about the simple fact that those areas are sensitive parts of our body. I am talking historic reasons here, not contemporary, in which case I believe religion has a strong impact.
I am back to reproduction and therefore partly sexuality. The stone age folks learned that it was prudent to cover those body parts as, if they got hurt or cold, they may very well impair reproduction and hence survival. I wonder if it actually is inherited genetically by the Evolution. Those who tended to cover their sensitive parts tended to have more children, who in their turn...etc...etc..
Of course, a more contemporary reflexion is that, otherwise we would not have the luxury of camel toe..
