http://www.nybooks.com/articles/17015
Splendid book review: Solitary Sex: A Cultural History of Masturbation. The reviewer (following his source) argues that modern demonisation of masturbation began during the Enlightenment. Because it was in its climactic moments deeply private, because it dealt with fantasy, and because it was thought to be insatiable it defied reason—and therefore was to be distrusted. (I don’t quite follow the argument here, but it’s worth reading anyway.)