One in five of the nation’s 646,000 same-sex couples consider themselves married, according to census figures released Tuesday showing a sharp rise in the number of gay people willing to identify themselves as couples.

Same-sex couples make up just 1 percent of the 64 million couples in the country, including married couples and unmarried partners, and barely half a percentage point of all households. But the number of same-sex couples increased 80 percent over those counted in the 2000 Census.

The figures represent couples who identified themselves to the census as spouses or unmarried partners living together. A post-census study by the Williams Institute at the UCLA School of Law found that 10 percent of same-sex couples did not reveal their relationship, even though the census is confidential. “The bulk of the increase is people being more willing to identify themselves,” said Gary Gates, a Williams Institute demographer who analyzes trends in the gay community. “There might be some increased partnering as the stigma declines, but that can’t explain it all.”

The count of about 131,000 married same-sex couples is almost certainly higher now, as New York state this year became the seventh, and largest, jurisdiction in the United States to legalize gay marriage. It had been legal in the District less than a month before the census was taken in April 2010 amid a campaign by gay activist groups to encourage couples to identify their relationships on the census forms.

The census statistics show same-sex couples in every state, including married couples living in states where their unions are not legally recognized. The District has the nation’s highest rate of same-sex couples, almost 2 percent of all households, though as a city its statistics are not really comparable to those of the states. About 750 of the city’s 4,800 same-sex couples are married.

About 12,500 same-sex couples live in Maryland, where a same-sex marriage bill was voted down this year. More than 14,000 live in Virginia, which does not recognize the legality of gay marriages performed out of state. Each state has about 2,500 married same-sex couples.

Reaction to the numbers

Proponents and opponents of same-sex marriage found solace in the census figures.

“It really shows us tremendous growth in the number of same-sex couples willing to stand up and be counted, and I hope it translates into governments being responsive to their needs,” said Brian Moulton, chief legislative counselor for the Human Rights Campaign’s marriage-equality advocacy efforts. “It’s a community that clearly exists all over the country and is not willing to be largely ignored by the folks who are supposed to represent them.”

Peter Sprigg, a senior fellow for policy studies at the Family Research Council, said the census figures show that the number of gay couples seeking to get married is small. He pointed to a census analysis that 42 percent of same-sex couples living in states where gay marriage is legal actually wed. In comparison, there are 54 million heterosexual married couples and 8 million couples who live together but are not wed.

“Even where it’s legalized, most same-sex couples living together do not choose to marry,” Sprigg said. “To my mind, this calls into question whether most homosexuals even want to participate in the institution of marriage.”

Sometimes the census’s categories fail to capture the complexity of the relationships it tallies.

Mark down Kathryn Hamm and her partner of 19 years, Amy, as one couple the census considers unmarried partners, even though they feel they have for all intents and purposes been married more than a decade.

They live in Arlington County, across the river from the District, and many of their friends have had weddings in the District, coming home to a state that will not recognize their union. They have not followed suit, Hamm said, allowing that it’s ironic — she was one of the first in her circle of friends to have a commitment ceremony in 1999 and now operates On her Facebook page, she lists her relationship status as “(not legally) married.”

“We live in Virginia, where same-sex marriage is constitutionally banned, so there would be no legal reason to get married,” she said. “And we feel that when we had our ceremony, we were truly married in the eyes of our family and friends, and spiritually bound through that ritual. No law on the books — or lack of law — can take that away.”

Many of Hamm’s clients come from Virginia. In fact, same-sex marriage is not legal in every one of the top five states where her clients reside, she said. If same-sex marriage becomes legal in Virginia, Hamm said, she and her partner will have a renewal ceremony and make things legal.

‘A huge debate’

Sharonda Purnell and her partner of three years, Carla, united in a marriage ceremony this month at their home in Ypsilanti, Mich. — a state that has a constitutional ban on same-sex marriage. Purnell wore a traditional white wedding gown, her partner an ivory tux, as they stood in a chapel before their pastor and 100 friends and said their vows.

Some of Purnell’s friends want to marry, too. But some, she said, quarrel with their lovers over whether to marry or not. “It’s a huge debate,” she said. “Often it’s causing a rift in the relationship about whether to marry or not. I lucked out. We both wanted to marry, and it was a beautiful wedding. Even the ones who are skeptical about getting married looked at it and said, ‘Maybe we should.’”

The census statistics on same-sex couples were the first since Massachusetts became the first state to legalize gay marriage and start issuing marriage licenses in 2004.

The numbers are only an estimate, however. The census initially said it counted 902,000 same-sex couples. But further analysis led the Census Bureau to revise the numbers markedly downward, not only for last year’s census but for 2000.

Martin O’Connell, chief of the fertility and family statistics branch of the census, blamed a faulty design in the questionnaire used when census enumerators came to the doors of people who did not mail their forms back. People asked to identify the relationship on the mail-in version were given a list of options in one column. Enumerators used forms that had the options spread over several columns.

About 28 percent of the couples identified as being the same gender in the 2010 Census, and more than 80 percent of those identified that way in the 2000 Census, were probably opposite-sex couples, O’Connell said. The figures were corrected based on a computer analysis of names written on the census, to determine what percentage of everyone with those names was a man or a woman.