It has been written that the Census Bureau has no data on same-sex couples; this is incorrect. Go to Families and Living Arrangements and scroll near the bottom. Not only will one find tables, one will discover a couple working papers that describe the political and statistical challenges about gathering such information. Gay marriage is a particularly complicated statistical issue.
Also, on American Factfinder, look for Table B11009. UNMARRIED-PARTNER HOUSEHOLDS BY SEX OF PARTNER in the American Community Survey.
There's a UCLA professor named Gary Gates, who has put together some statistics based on 2000 Census data and other sources, including the Gay and Lesbian Atlas.