Theories of Gender are recent. The fact that we are now aware that society socially constructs our differences, is just the extent and evolution of the discipline (sociology, anthropology and psychology), as well as the result of feminist movement and its influence.
Gender is a relatively recent word we use to define and describe such discrimination. For an understanding of the genealogy of sex/gender, you should refer to Simone de Beauvoir.
Nowadays, Gender Theory literature is huge: among the most important contributors you should read Judith Butler. Sex and gender are complementary and the correct use of the term depends on what you want to point out: biology, social construction or choice?