The US Supreme Court tackled same-sex unions for a second day Wednesday, hearing arguments for and against the 1996 US law defining marriage as between one man and one woman.
After the nine justices mulled arguments on a California law outlawing gay marriage on Tuesday, they took up a challenge to the constitutionality of the federal Defense of Marriage Act (DOMA).
The 1996 law prevents couples who have tied the knot in nine states -- where same-sex marriage is legal -- from enjoying the same federal rights as heterosexual couples.
The plaintiff is Edie Windsor, 83, who was ordered to pay federal inheritance taxes of $363,000 following the 2009 death of Thea Spyer, her partner of more than 40 years. The couple had married in Canada in 2007.
The surviving half of a heterosexual couple would not have faced the same tax demand. Windsor is challenging Section 3 of DOMA on the grounds it is discriminatory because it defines marriage as between a man and a woman.
Up to 800 people gathered outside the court Wednesday, with a majority shouting slogans and carrying placards in support of marriage equality. On the opposing side, a poster read: "God hates gay marriage."
President Barack Obama's administration had opposed Windsor's bid to repeal Section 3 as it progressed through the lower courts, where the legislation was twice ruled as unconstitutional.
But the White House has since switched sides. Now it is calling for the law to be overturned, leaving DOMA to be defended by a group of Republican lawmakers, along with a coalition of religious and conservative groups.
"The case is pretty simple. It's about discrimination," said James Esseks, one of Windsor's lawyers.
"It doesn't make sense in America for a federal government to treat two different people, married under the same state law, different ways. That is unfair, it is un-American and it should be unconstitutional."
Elizabeth Wydra, chief counsel for the Constitutional Accountability Center, added: "With respect to DOMA, it's an irrational federal discrimination against legally married same-sex couples.
"I think it is a pretty simple and clear case."
Opponents of Windsor's case, however, are contesting the portrayal of homosexuals as the victims of discrimination, describing them in a brief as "one of the most influential, best-connected, and best-organized groups in modern politics."
Some legal experts say the Supreme Court may decide that DOMA interferes with the right of each of the 50 states to set down its own marriage laws.
"It may sound to some justices like DOMA infringes on federalism or states' rights," said David Cruz, a professor at the University of Southern California.
The court trod cautiously Tuesday as it considered the constitutionality of California's Proposition 8, the ballot initiative through which in 2008 voters ended the right to same-sex marriage in the nation's most populous state.
While it will take months to issue a ruling, several justices indicated they would be in no hurry to make a verdict that could extend the right to same-sex marriage to the entire country.
Justice Anthony Kennedy, whose remarks are closely watched as he is often the swing vote on the bench, voiced reluctance about the court stepping into "uncharted waters" on a case involving Californian law.
Justice Samuel Alito, a conservative, spoke of a "lack of data" on same-sex marriage since it was first legalized in the Netherlands in 2001.
Successive public opinion polls have indicated that a majority of Americans now accept the principle of same-sex marriage, including an overwhelming number of younger citizens.
In May last year, Obama became the first serving US president to publicly back gay marriage.