gaydemographics.org provides data about same-sex couples drawn from the 1990 and 2000 decennial censuses and from the 2002 American Community Survey. From the 2000 Census, the number and the percentage of same-sex couples is provided for each U.S. state:
U.S. Census Bureau Reports:
Married-Couple and Unmarried Partner Households: 2000
Hispanic Origin and Race of Coupled Households provides data about same-sex unmarried-partner households for 2010 (CPH-T-4) and 2000 (PHC-T-19).
Technical Note on Same-Sex Unmarried Partner Data From the 1990 and 2000 Censuses
Unbinding the Ties: Edit Effects of Marital Status on Same Gender Couples (April 1999)
American FactFinder 2 (U.S. Census Bureau)
American Community Survey, Table B11009, Unmarried Partner Households by Sex of Partner
Census 2010 Summary File 1, Table PCT15, Husband-Wife & Unmarried-Partner Households by Sex of Partners by Presence of Related & Own Children Under 18 Years
Census 2000 Summary File 1, Table PCT14, Unmarried Partner Households by Sex of Partners
Census 2000 Summary File 2, Table PCT22, Unmarried Partner Households and Sex of Partners (race, ethnicity)
Census 2000 Summary File 3, Table PCT1, Unmarried Partner Households by Sex of Partners
Census 2000 Summary File 4, Table PCT21, Unmarried Partner Households by Sex of Partners (race, ethnicity, ancestry)