More than 200 businesses will urge the U.S. Supreme Court on Wednesday to strike down a federal law that restricts the definition of marriage to heterosexual unions.
Lawyers representing the businesses said they would file a brief in the case.
Companies including Microsoft Corp, Google Inc, Starbucks Corp and Pfizer Inc are among those that joined the brief. Others included Aetna Inc, Amazon.com, Inc and Citigroup Inc.
Thomson Reuters Corp is another signatory. The Reuters news agency is part of Thomson Reuters.
The companies want the Supreme Court to strike down a key provision of the federal Defense of Marriage Act (DOMA) that defines marriage as a union between a man and a woman.
Separately, lawyers representing another group of employers, including some of the same companies, had said already that they planned to file a brief on Thursday in a related case that questions a California law that bans gay marriage.
The two cases are to be argued before the Supreme Court on March 26 and 27.
In the brief filed on Wednesday, attorney Sabin Willett wrote that DOMA "requires that employers treat one employee differently from another, when each is married, and each marriage is equally lawful."
DOMA does not create any uniformity nationwide, Willett said, because 12 states in total either authorize same-sex marriage or recognize marriages that have been performed in other states.
That creates a burden for employers, particularly those who do business nationwide, he added.
Willett also wrote that the law forces companies to discriminate, sometimes in contravention of their own internal policies and local laws, when dealing with healthcare plans and other benefits.
"We must do all of this in states, counties and cities that prohibit workplace discrimination on the basis of sexual orientation and demand equal treatment of all married individuals," he added.
In briefs already filed in support of marriage being restricted to heterosexual unions, business interests have not been represented. The U.S. Chamber of Commerce has not taken a stand on the issue.