A new coalition of large, national businesses is joining the battle for same-sex marriage.
The Human Rights Campaign announced the launch of the Business Coalition for DOMA Repeal this week, which aims to abolish the Defense of Marriage Act, a 1996 law that defines marriage as between a man and a woman.
Thirteen businesses have signed on so far, including Armani Exchange, Aetna (, )eBay (, )Thomson Reuters ( and )Marriott International (, where former Republican presidential candidate )Mitt Romney is a board member.
The coalition specifically supports the Respect for Marriage Act, which was first introduced in 2009 and is expected to be reintroduced in Congress next month.
By repealing DOMA, the act would extend more than 1,000 federal benefits to same-sex married couples that currently only opposite-sex couples receive. These include the ability to file taxes jointly, receive Social Security survivor benefits and qualify for certain estate and gift tax exemptions.
The Human Rights Campaign said that not only is DOMA unconstitutional, but it's bad for businesses -- causing "administrative headaches and tax inequities for companies as they simply try to treat their employees fairly."
For example, same-sex couples are required to pay federal income tax on health benefits provided to a spouse through an employer-sponsored health insurance plan. Some employers reimburse employees for the extra tax paid, which requires extra time and money.
In addition to the Respect for Marriage Act, the Supreme Court also announced in December that it will review the constitutionality of DOMA for the first time.
Meanwhile, other businesses are voicing their support for same-sex marriage on a state level. In an open letter to Illinois lawmakers earlier this month, 50 business leaders and companies said "it is vitally important that Illinois lawmakers enact marriage equality soon" and cited a Williams Institute study showing that same-sex marriage would generate millions of dollars in new revenue for Illinois businesses.
Google (, )Orbitz ( and )Groupon ( were among the bigger companies to sign the letter, along with CEOs from )Hyatt Hotels (, )Morningstar ( and )Exelon (. )