The binary gender system in many nations, including the united states, means that in order to gain certain legal recognition as an individual, one must identify as

Nations which uses the binary gender system requires a person or an individual to identify only either as a "male or female".

This type of social system usually forms the basis for how one is educated, what jobs one can do (or are expected to do), how one is expected to behave, what one is expected to wear, and etc.