That means nothing to the federal government, which recognizes only a marriage between a man and a woman. Federal authorities ordered Tan, who is not a U.S. citizen, to be deported from San Francisco to her native Philippines this morning.
But with the help of her congresswoman, Rep. Jackie Speier, D-Hillsborough, Tan received a three-week stay of the deportation order, allowing her to remain with her family until April 22.
"This is good news, since we now have a little more time for the legal process to work," said Tan's attorney, Phyllis Beech. "If this was not a same-sex couple, the spouse of an American citizen would be able to immigrate fairly quickly and fairly easily. Instead, they're being discriminated against because they are lesbians."
Under the federal Defense of Marriage Act, domestic partnerships and even legal same-sex marriages, such as those last year in California and which still occur in Massachusetts, aren't recognized as valid in immigration cases.
"I just really feel helpless," Mercado said. "If I was a man, none of this would be happening."
A spokeswoman for U.S. Immigration and Customs Enforcement said the agency does not comment on individual cases.
For Tan, 43, the whole affair is a shock. After meeting Mercado in the United States in 1986, she returned from the Philippines on a visitor's visa in 1989 to be with her lover.
In 1995, she applied for political asylum, saying she feared for her life in her home country. She had been shot nearly two decades earlier by a relative in a dispute over an inheritance.
"I was 14 when it happened, and I was shot in the head and beaten," she said. "It's a miracle that I'm alive."
Any deportation action was stayed while the asylum request was being considered. But when the Board of Immigration Appeals turned down the request in 2002 and ordered Tan deported, no one - not even her previous attorney - told her.
"She and her family even received a security clearance to tour the White House in 2005," said Beech, a Fresno attorney who took over Tan's case this year. "They checked her fingerprints and never said there was a deportation order outstanding."
The respite ended Jan. 28, when immigration agents showed up at Tan's home in Pacifica to take her into custody.
"I'm not an activist," Tan said. "We're just common people living a peaceful and happy life, and then this happens."
It's a familiar story, said Rachel B. Tiven, executive director of Immigration Equality, which works to end discrimination in immigration law against lesbian, gay, bisexual, transgender and HIV-positive people.
"We hear every day from American citizens whose families are being torn apart because their partner is being deported," she said.
A survey commissioned by Immigration Equality found that in 2000 there were about 37,000 same-sex couples in the country where one partner was a foreign national. About half of these couples had children under 18.
"The average age of these people is 38, so we're talking about long-term relationships," Tiven said.
Federal officials can look past immigration violations when heterosexual couples have children, Beech said.
"Even on an overstay (like Tan's), Congress has always taken the position that maintaining the family unit is so important that a noncitizen spouse can be legalized virtually without penalty," she added.
Since 2000, Rep. Jerrold Nadler, D-N.Y., has introduced bills to allow the "permanent partners" of U.S. citizens the same avenue to permanent resident status spouses now have.
The bills have never made it out of committee, but that could change, said Ilan Kayatsky, a spokesman for Nadler.
"The landscape is changing with a president who supports the idea of immigration change and more progressives in both chambers," he said.
So far, 93 members of the House and 17 senators have signed on as co-sponsors of either HR1024, the Uniting American Families Act of 2009, or its companion measure in the Senate, S424 by Sen. Patrick Leahy, D-Vt.
The stay of the deportation order opens some paths for Tan. Her attorney is still working on efforts to get the appeals board to reverse its decision, and Sen. Dianne Feinstein's office is reviewing the case to see if anything can be done for Tan.
But those efforts might not be enough to keep her off that plane to the Philippines later this month. And if that happens, her entire family will pull up their roots to follow.
"We'll move, and we'll be together," Mercado said. "We raised our family, and we won't let anyone tear us apart."