The gender binary is the idea that human gender exists in two forms: masculine and feminine. The term also describes the system in which a society divides people into male and female gender roles, gender identities and attributes. Gender role is one aspect of a gender binary. Every known society has used the gender binary to divide and organize people, though the ways this happen differ among societies. A universal aspect of the gender binaries is that women give birth. Gender binaries exist as a means of bringing order. Certain notable religions are often used as authorities for the justification and description. Islam, for example, teaches that mothers are the primary care givers to their children and Catholics believe only males may serve as priests.
Exceptions have widely existed to the gender binary in the form of transgendered people. Besides the biological identification of intersexuals, elements strictly of the opposite sex have been taken by people biologically female and male such as two-spirited Native Americans and hijra of Indians. In the contemporary West, transgendered break the gender binary in the form of genderqueer, drag queens, and drag kings. Transsexuals have a unique place in relation to the gender binary because they transition from one side of the gender binary to the other.
The terms androgyny, intergender, bigender, multigender, third gender, neuter/neutrois/agender, and gender fluid may also be used to describe where one lies on a gender spectrum or in gender spheres outside of the normal binary genders.
Source: http://en.wikipedia.org/wiki/Gender_binary
