The Census Bureau says the federal Defense of Marriage Act bars the agency from recognizing gay marriages in the nation’s 10-year count, even though the marriages are legal in Massachusetts and California.

The agency’s director, Steven Murdock, said in an interview Thursday that the 1996 federal law “has that effect, in terms of being a federal agency. We are restricted by it.”

The Census Bureau does not ask people about their sexual orientation, but it does ask about their relationships to the head of the household. Many gay couples are listed in census figures as unmarried, same-sex partners, though it is an imperfect tally of all gay couples.

Murdock said the bureau will strive to count same-sex couples in the 2010 census, just as it has in the past. But those people who say they are married will be reclassified as unmarried, same-sex partners.

Same-sex couples with no children will not be classified as families, according the bureau’s policy. Those with children who are related to the head of the household will be classified as families.

Gay rights advocates complained that the Census Bureau is depriving them of a hard-fought legal recognition.

“To completely whitewash us out of existence is hurtful, discriminatory and shameful,” said Molly McKay of Marriage Equality USA, a California-based group that advocates for same-sex marriage. “It’s like the federal government is trying to say that we don’t exist.”

McKay said an accurate count of same-sex married couples would help policymakers determine the costs of providing benefits.

McKay, 38, said she plans to marry her partner of 12 years on Sept. 1, now that they are legally able to marry in California. She said they consider themselves “an old married couple,” even if the government doesn’t.

“This is a very sweet moment in our life. It really is an absolutely joyous time,” McKay said. “The notion that the federal government is going to come in and erase our existence is un-American.”

The Census Bureau is required by the Constitution to conduct a comprehensive count of the nation’s residents every 10 years. Every question is either mandated by federal law or used to administer a federal program, Murdock said.

Same-sex marriage was not an issue in the 2000 census because it wasn’t legal in any state. The Census Bureau’s policy on same-sex marriages was first reported in the San Jose (Calif.) Mercury News.

The bureau relies almost entirely on people’s responses to classify them by race, ethnicity, age and income. But not marital status – at least not in 2010.

“It really should be what you say you are, not what I perceive you to be,” Murdock said. But, the agency director added, “We have some limitations. This particular act limits us in regards to this issue.”