Starting in 1990, the U.S. Government began including an “Unmarried Partner” status on its decennial census forms.  This change allowed non-married couples in committed relationships to record this fact on census forms.  Not surprisingly, many same-sex couples took advantage of the new category to record their relationships on the 1990 census.  Approximately, 150,000 couples were counted in the “Same-Sex Unmarried Partner” category in 1990.  For a good analysis of the census information about these couples, see the National Gay and Lesbian Task Force’s document Same-Sex Unmarried Partners: Analysis of Data from the 1990 U.S. Census [PDF file].

Virtually everyone agreed that same-sex unmarried partner households were grossly undercounted in the 1990 census.  Although fear of discrimination might have prevented some couples from identifying their relationship on census forms, others were not counted because they chose to identify themselves as “spouses” rather than as “unmarried partners.”  The government reassigned these couples to other categories.  An article from the Bergen Record explains what happened:

Back in 1990, same-sex couples who identified as ``spouses'' -- a legal impossibility -- were quietly ``reallocated'' to more traditional relationships.

Instead of being counted as gay couples, many were reclassified as siblings or roommates. If their age difference exceeded 15 years, they were reclassified as parent and child. Most, however, faced an even bigger change.

``The most likely case was they were turned into opposite-sex married couples,'' said Greg Spencer, chief of the Census Bureau's population projections branch.

Only a minority survived as ``unmarried partners.''

(http://www.contemporaryfamilies.org/public/articles/change15.htm)

The Census Department released a Technical Note describing its policies for reassigning same-sex couples to other categories in the 1990 census.

For the 2000 census, the federal government altered its handling of same-sex couples who checked off the “spouse” category to describe their relationship:  these individuals were automatically assigned to the “unmarried partner” category.  This new approach allowed for more accurate reporting of same-sex couples in the 2000 census, but it made it difficult to compare data from the 1990 census to the 2000 census numbers.

To both counter fears of discrimination and to prevent the problems that plagued the 1990 census, national gay and lesbian activist groups launched a “Make Your Family Count!” campaign, which encouraged gay and lesbian couples to use the “Unmarried Partner” category to describe their relationship.