Sexy is okay if it's covered in the sheen of "glamour". But if it's stripped bare, to its natural essence, it is no longer acceptable. It's somehow too stark and real, and we can't face it. Without all the layers of post-processing, it reminds us of amateur porn.
Natural nudity is okay as long as it's unsexy, but there's something uncomfortable about the natural eroticism of the human body. Frankly, I think natural sexuality is far more beautiful than the flashy accessories of the glamour culture. But why is it less pure? Because it is more direct?
Why is the earring more acceptable than the ear? The lipstick more acceptable than the lip? The stocking more acceptable than the leg? We are permitted to fetishize sexuality because it distracts our attention away from the source of what is sexy, onto a mere symbol. But all this attitude does is reflect on how uncomfortable we are with our sexuality. Our true, raw sexuality.
