How the Census Works
The Census Bureau strives to contact every person living in the United States with the Census. In order to do so, the survey must be accessible to everyone and people must be open to answering the questions included on it.
As seen below, the questions primarily address number of people in a household, their sex the U.S. census does not currently include options outside "male" and "female". From the U.S. Census Bureau website: "At the Census Bureau, the sex question wording very specifically intends to capture a person's biological sex and not gender. Ambiguity of these two concepts interferes with accurately and consistently measuring what we intend to measure--the sex composition of the population" and their race "data are based on self-identification. People may choose to report more than one race group. People of any race may be of any ethnic origin" (U.S. Census Bureau).