-- Some of the nation's most prominent corporations have joined the legal battle against the federal ban on marital benefits for same-sex couples, saying the law drives up employers' costs and makes them the agents of government-sanctioned bias.

The 1996 law, known as the Defense of Marriage Act or DOMA, "conscripts (employers) to become the face of its discrimination," lawyers for 60 companies said Thursday in papers filed with a federal appeals court in Boston.

Participants included Microsoft, Google, Aetna, Nike, Levi Strauss, Starbucks, CBS and Time Warner Cable. The brief was also signed by several trade organizations and the city governments of Boston, New York and Cambridge, Mass.

DOMA denies federal benefits, including joint tax filings, Social Security survivor coverage and immigration privileges, to same-sex couples legally married in their states.

A federal judge in Boston declared the law unconstitutional in July 2010, saying the federal government had historically respected states' definitions of marriage and refused to do so in this case because of prejudice.

After President Obama withdrew his administration's defense of DOMA in February, House Republicans hired Paul Clement, a former U.S. solicitor general, to argue on behalf of the law in about a dozen cases around the nation, including two in the Bay Area. The Boston case is the only one that has reached an appeals court.

The corporations' brief did not discuss constitutional issues that Clement is debating with gay rights advocates and instead focused on the law's impact on employers, something the court could consider in deciding whether its benefits outweigh its burdens.

The costs include higher taxes on employers that provide health benefits to their employees' same-sex spouses, the companies said. Federal law taxes both employers and employees on the value of those benefits but exempts benefits for opposite-sex spouses, the brief noted.

The companies also said many employers provide additional benefits to gay and lesbian employees to equalize their treatment in the workforce. For example, a company, at its own expense, might offer family leave to care for an ailing same-sex spouse, or beef up its retirement program to match the survivors' benefits available to opposite-sex couples.

By requiring employers to treat some workers as married under state law but single under federal law, DOMA forces them to keep "two sets of books" and undergo administrative burdens, the brief said.

It said the law also damages workplace morale.

"The employer becomes the face of DOMA's discriminatory treatment, and is placed in the role of intrusive inquisitor (and) withholder of benefits," the companies' lawyers told the court.

As of 2008, they said, 94 percent of Fortune 500 companies protected their gay and lesbian employees against discrimination. DOMA, the lawyers said, requires that "we renounce these principles, or betray them."

E-mail Bob Egelko at begelko@sfchronicle.com.