The gender binary is a term used to describe the idea that there are only two possible genders, man and woman, which can be assigned on the basis of genitalia. This system often also enforces masculinity for men and femininity for women through gender roles and socially-imposed restrictions to gender expression and sex characteristics.
Although many people consider the gender binary to be an accurate description of gender, there are many who identify as genders outside of male and female, which are referred to as non-binary. Additional genders have a long history in some non-Western cultures. However, in cultures which follow the gender binary, non-binary people are often systematically excluded. This exclusion may also extend to people who identify as transgender men or women, for failing to have a gender identity which matches their assigned sex.