Seattle has joined seven other U.S. cities and dozens of corporate employers in the ongoing constitutional challenge to the federal government's Defense of Marriage Act (DOMA), which limits marriage to a one-man, one-woman union.

The eight cities, and the companies that Tuesday filed the friend-of-the-court brief with them — including Starbucks and Microsoft, are all in states where a same-sex-marriage law exists or, as is the case in Washington state, where such laws are on hold.

They argue in the brief, filed in the 9th U.S. Circuit Court of Appeals, that DOMA forces them as employers to create two separate groups of married employees — gay and straight — whom they are required to treat unequally when providing certain forms of workplace benefits, primarily health care.

At a news conference Tuesday, Seattle City Attorney Pete Holmes joined Mayor Mike McGinn and Councilmember Tom Rasmussen to explain that Seattle's decision to join is not just about fairness but also about cost.

Many employers in states with same-sex-marriage laws — and those in states like Washington with domestic-partnership laws — must provide health-care benefits to all employees, regardless of their sexual orientation.

Under DOMA, the federal government treats health-care benefits to an employee's same-sex partner as income, subject to federal income taxes — a cost not incurred by opposite-sex couples.

As an employer providing benefits to employees in same-sex partner relationships, Seattle taxes gay employees the value of their partners' health insurance.

Seattle and the other employers also incur payroll taxes on the value of those benefits as if they were wages to the worker.

About 150 Seattle city employees get health-insurance coverage for their same-sex partners, at an annual payroll cost to the city of about $83,000, city officials say.

"We want to treat the health insurance we provide to same-sex domestic partners and families just like we do for opposite-sex married people, so that we can free up more money to put to other uses," Holmes said.

"But we've also signed on to the brief for the simple reason that we believe government should treat everyone the same."

Currently, 10 states and the District of Columbia either authorize same-sex couples to marry or recognize same-sex marriages from elsewhere. Governors in Washington and Maryland this year signed same-sex-marriage bills that will be subject to a referendum vote in November.

Among other things, the Defense of Marriage Act, passed by Congress in 1996, prohibits federal agencies from recognizing same-sex couples as legally married, regardless of the laws in their home states.

The suit at issue — one of two challenging DOMA — was filed two years ago against the U.S. Office of Personnel Management by Karen Golinski, a Court of Appeals worker in San Francisco who was denied spousal health benefits because of DOMA.

In February 2011, the Obama administration called DOMA unconstitutional and stopped defending it in court. Republican House Speaker John Boehner convened the Bipartisan Legal Advisory Group, or BLAG, to defend it.

In February, a U.S. District Court judge declared DOMA unconstitutional as applied to Golinski's case. BLAG appealed, and oral arguments in the 9th Circuit are set for the week of Sept. 10.

In the brief Seattle signed, dozens of financial institutions, health-care providers, technology firms as well as the cities of Boston, Cambridge, Mass., Los Angeles, New York, San Francisco, and Santa Monica and West Hollywood, Calif., argued against DOMA.

As employers, they say providing robust workplace benefits is crucial to attracting and keeping the best and the brightest workers. DOMA, with its uneven treatment of married couples and the financial penalty it imposes on employees in such unions, complicates that.

"In the modern workplace, the employer becomes the face of DOMA's discriminatory treatment," the brief said.

Lornet Turnbull: 206-464-2420 or lturnbull@seattletimes.com. On Twitter @turnbullL.