When the U.S. Census Bureau counts same-sex married couples next year, demographers expect hundreds of thousands to report they are spouses — even though legal same-sex weddings in the United States number in the tens of thousands.

Gay advocates say they plan to use “A Census That Reflects America’s Population,” as the Census Bureau calls its plan to report same-sex marriage statistics, to push for legislative and policy initiatives, while groups opposed to same-sex marriage weigh a counteroffensive.

Particularly at the state and local levels, gay advocacy groups say census data on income for same-sex couples will show the need for more protections against job discrimination. Statistics on households with children will help them challenge laws limiting gay adoptions and legal guardianship. With raw numbers to illustrate the need, it will be easier to demand services, they say.

“Why does the census ask if people are young or old, black or white, married or single?” said Joe Salmonese, president of the Human Rights Campaign, which promotes civil rights for gay, lesbian, bisexual and transgender people. “It’s because we want to understand if the country is meeting the public-policy needs of those Americans. That’s particularly so for LGBT Americans.”

But conservatives say the tally could just as easily support their position that most gay people aren’t looking to get married. They say they will oppose attempts to make policies more gay-friendly.

“It seems homosexual activists use these various markers as ways to push their agenda, to force people to go along with whatever they demand,” said Wendy Wright, president of Concerned Women for America.

The 2010 Census will not be the first in which same-sex couples have identified themselves as married. But it will be the first in which the raw numbers are publicly reported, reflecting an evolution in the way the Census Bureau keeps track of American lifestyles.