The presently ubiquitous idea of how gender is defined remains deeply indebted to modern thought, depending on the concept of sex as its reference point. Originating with practices arising from capitalism, modern thought’s concept of sex was enhanced with the search for the norm regarding standardization, then further refined through the research program of eugenics. Gender, in contrast to sex, has its roots in sexual practice changes from the second half of the twentieth century onward, developed in association with 1980s postmodern thought. The presuppositions of the two concepts differ radically. Understanding the difference between sex and gender matters because gender is now more than a critique of sex—it is an idea that is accepted and understood in popular culture and affects how people interpret their sexuality. This review examines the foundations of modern and postmodern thought, then focuses on sex and gender with respect to their history within these modes of thought. The conclusion, pointing to future research, is that a further reformulation of sexuality will be required for gender, as a postmodern idea, to truly separate itself from the presumptions of sex as a modern concept, permitting individuals to express their sexuality as they choose without negative social and psychological repercussions.