Because society, which is a mask because nobody really lives according to it. Makes everything into a taboo. We were born naked, we didn't knew what shame was until we learned it from out parents. If you were born in a jungle you wouldn't even care. It's all about how "society" sees it and wants you to be.
Or if you're religious you can always blame Adam and Eve for eating the forbidden fruit and thus they could now see what was "wrong" and "right" they learned that they were always naked in the presence of God and felt ashamed. If they hadn't eaten the fruit we would all be around happily naked without lewd thoughts.
Or if you're religious you can always blame Adam and Eve for eating the forbidden fruit and thus they could now see what was "wrong" and "right" they learned that they were always naked in the presence of God and felt ashamed. If they hadn't eaten the fruit we would all be around happily naked without lewd thoughts.