Like any form of behaviour, sexuality could have its origins in any or all of three sources: the biology of human organisms; the psychology or intellect of individuals; and the environment, either physical, socio-cultural, or both. Which of these is identified as the primary source of sexual behaviour is linked with the way sexuality is defined and the way possibilities for controlling its expression are conceived. Few people presently believe that sexuality originates in individual psychology or intentions. Rather, most opinion is divided between two other, alternative views. The first holds that human sexual behaviour is produced by human biological nature. The second alternative asserts that sexuality is a product of the socio-cultural environment. Given the present state of our understanding of sexuality, it is not possible to proclaim either of these two opinions about the origins of sexual behaviour to be correct. I believe that there are good reasons for favouring the socio-cultural conception of sex. In this discussion, however, I wish only to emphasize the importance to proposals for sex education of the controversy over these two views, and to explore aspects of the socio-cultural view that are relevant to judging the educational legitimacy of current proposals for school-based sex instruction.