At the end of the twentieth century, in response to the international movement for gender equality, various measures were created to eliminate gender discrimination in Japan: the Basic Law for Gender-Equal Society was enacted in 1999, and a variety of policies were developed. Since the beginning of the twenty-first century, however, a backlash against gender issues by conservative cultural figures, politicians, and the religious right has emerged. At issue was “gender-free” education. A campaign of lies, such as “they are forcing boys and girls to change clothes in the same room in the name of gender freedom,” completely halted the movement toward gender equality in Japan. Conservative forces that criticized gender-free education began to target the abolition of the concept of gender itself. The target of criticism at that time was John Money, a sexologist who popularized the gender concept along with the New Guinea research of Margaret Mead, a cultural anthropologist who discovered that the temperaments of men and women change depending on culture. In criticizing Money, backlashers referred to the work of Milton Diamond, who also criticized Money, arguing that his perspective of gender as a socially constructed sex was false. However, although Diamond was critical of Money’s research methods, he clearly insisted on the perspective of gender as created by the social environment. For Diamond, the perspective of analyzing gender from the perspective of both gender as a social construct as well as biological sex differences (i.e., Nature and Nurture) was obvious. Nevertheless, the backlashers sought to distort Diamond’s argument and deny the very concept of gender as a social construction. Diamond’s position could be summed up in his slogan, “Nature loves diversity.” When we consider sex, we need to analyze the diversity of sexuality from both the perspective of sex, the biological sex difference, as well as the perspective of gender, which is deeply related to society and culture.