The human species considered from the standpoints of comparative anatomy, physiology, pathology and bacteriology

Chapter Three begins by taking up the distinctive features of human social organization and of sexual anatomy, physiology, and behaviour. As already mentioned, features that make us strange among animals include our societies of nominally monogamous couples, our genital anatomy, and our constant and generally private pursuit of sex. Our sex lives are reflected not only in our genitalia but also in the relative sizes of men's and women's bodies (much more equal than are the bodies of male and female gorillas or orangutans). We shall see how some of these familiar and distinctive features have known functions, while others continue to defy understanding. No honest discussion of the human life-cycle could get away with noting that we are nominally monogamous and just leaving it at that. Pursuit of extramarital sex is obviously greatly influenced by each individual's particular upbringing and by the norms of the society in which the individual lives. Despite all that cultural influence, we are left with having to explain the facts that both the institution of marriage and the occurrence of extramarital sex have been reported from all human societies; but that extramarital sex is unknown in gibbons, although they do practise 'marriage' (that is, lasting male/female pairing to rear offspring); and that the question of extramarital sex is meaningless for chimpanzees because they do not practise 'marriage' ...»