" She is distinguished from a courtisane only in that she does not offer her body for money by the hour like a commodity, but sells it into slavery for once and all. Fourier's words hold good with respect to all conventional marriages: "As in grammar two negatives make one affirmative, so in matrimonial ethics, two prostitutions are considered as one virtue." Sexual love in man's relation to woman becomes and can become the rule among the oppressed classes alone, among the proletarians of our day—no matter whether this relation is officially sanctioned or not. Here "

― Friedrich Engels , The Origin of the Family, Private Property and the State