In her most influential book Gender Trouble (1990), Butler argued that feminism had made a mistake by trying to assert that 'women' were a group with common characteristics and interests. That approach, Butler said, performed 'an unwitting regulation and reification of gender relations' -- reinforcing a binary view of gender relations in which human beings are divided into two clear-cut groups, women and men. Rather than opening up possibilities for a person to form and choose their own individual identity, therefore, feminism had closed the options down.
Butler notes that feminists rejected the idea that biology is destiny, but then developed an account of patriarchal culture which assumed that masculine and feminine genders would inevitably be built, by culture, upon 'male' and 'female' bodies, making the same destiny just as inescapable. That argument allows no room for choice, difference or resistance.

Butler prefers 'those historical and anthropological positions that understand gender as a relation among socially constituted subjects in specifiable contexts'. In other words, rather than being a fixed attribute in a person, gender should be seen as a fluid variable which shifts and changes in different contexts and at different times. http://www.theory.org.uk/ctr-butl.htm