2000 Census information on same-sex couples, urban/rural definition
This is how the Census Bureau defines an urban area:
1. An urbanized area consists of densely settled territory that contains 50,000 or more people.
2 The Island Areas are American Samoa, Guam, the Northern Mariana Islands, and the Virgin Islands of the United States.
3 An urbanized area delineated as a result of a special census conducted by the Census Bureau during this decade (an intercensal urbanized area), at the request and expense of local governments, will be qualified using these criteria and the population counts reported in that special census.
4 An urban cluster consists of densely settled territory that contains at least 2,500 people, but fewer than 50,000 people. Major airports adjoining qualifying urbanized areas and urban clusters are those airports that, according to 2000 Federal Aviation Administration statistics, had an annual enplanement of at least 10,000 people, and thus qualified as a primary airport in that year.
More information here.