What took Japan so long? Erotic paintings and sculpture's have been around for thousands of years and many are deemed to be priceless. Modern erotic photographs are simply the extension of an artistic viewpoint using modern technology. Erotica has proven itself to be a viable art form throughout history. Art makes people think and gets them talking and erotic photographs meet that qualification.
One mans junk is another mans treasure. Naked portraits have been painted and sculpted for several millenia. They have been considered art. What determines the distinction between smut and art? The reasonable thing to do is call it all art, and let individuals decide what they believe. If you go to a museum and ask a group people to give their opinion on a particular piece of art, a few will think it's beautiful, and some will think it's chicken scratch. Pornographic images are not different, they are a form of art.