See Friedrich Engels (philosophical collaborator of Marx): In capitalism, marriage degenerates "often enough into the crassest prostitution"; the married woman "only differs from the ordinary courtesan in that she does not let out her body on piece-work as a wage earner, but sells it once and for all into slavery" (p. 82). This is from his 1884 book, The Origin of the Family, Private Property, and the State. New York: International Publishers, 1942. Peking, China: Foreign Languages Press, 1978. The other locus classicus is this 1917 essay by Emma Goldman: "The Traffic in Women," in the collection Anarchism and Other Essays. New York: Dover, 1969, pp. 177-94. An nice overview of the issue is provided in Alison Jaggar's "Prostitution," in Alan Soble, ed., The Philosophy of Sex: Contemporary Readings, 1st edition. Totowa, N.J.: Littlefield, Adams, 1980, pp. 348-68. Not only Marxists and Feminists have made the point (or drawn the comparison); sociobiologists and evolutionary psychologists who argue that women trade sex for resources in marriage also make it. See, for example, Donald Symons, The Evolution of Human Sexuality. Oxford, Eng.: Oxford University Press, 1979. This territory is covered in several entries in Sex from Plato to Paglia: A Philosophical Encyclopedia (Greenwood Press, 2006).