Following up on KQ's points: A few years ago I heard Gloria Steinem, one of history's great champions of women's rights, on NPR's
Fresh Air. Steinem spoke of the distinction between erotica (which celebrates the human form and lovemaking) and pornography (which demeans the female human form and fucking). Steinem said she sometimes views erotica and enjoys it, while she abhors pornography. Her argument was lucid and persuasive.
Not all nudity is pornography. Indeed, most nudity is not pornography. Our culture's collective repression and puritanical shaming of our natural naked state enables pornographers. When we view the female form as beautiful and something to be celebrated and admired, we disarm the pornographers.
Of course, the line between erotica and pornography is a subjective one, thus there's no practicable way to construct a rule allowing the former and banning the latter.
ETA: Link to Gloria Steinem's article entitled
Erotica and Pornography: A Clear and Present Difference, first published in
Ms. Magazine:
http://www.angelfire.com/sc3/ws301/html/steinem.html