The law does not prohibit states from allowing same-sex marriages, but it also does not force states to recognize them from other states. Most of the current plaintiffs are federal workers, who are not allowed to add their spouses to health care plans, and other benefits.

Many other states, including New Jersey, Illinois, Delaware, Rhode Island and Hawaii, have legalized domestic partnerships and civil unions for such couples -- a step designed in most cases to provide the same rights of marriage under state law.

But other states have passed laws or state constitutional amendments banning such marriages. California's Prop 8 is the only such referendum that revoked the right after lawmakers and the state courts previously allowed it.

In February, a federal appeals court in San Francisco ruled the measure unconstitutional.

Court could offer final word

By patiently letting legislatures and the voters decide the social and practical implications of same-sex marriage over the past decade, the high court is now poised to perhaps offer the final word on the tricky constitutional questions.

The split 5-4 conservative-liberal bench has the option of ruling broadly or narrowly-- perhaps taking a series of incremental cases over a period years, building political momentum and public confidence in the process.

Edie Windsor is one of more than a dozen plaintiffs involved in the current DOMA appeals before the high court. She and Spyer, a psychologist, had been a devoted couple in New York's Greenwich Village for more than 40 years, before marrying in Canada in 2007.

New York did not allow same-sex marriages to be performed in the state when Windsor and Spyer wed, but did recognize the out-of-state license. New York's legislature last year approved same-sex marriage.

Windsor, a retired computer systems programmer, wants the $363,053 in added estate taxes she was forced to pay the IRS. She said the federal government considers her relationship with Spyer as little more than "girlfriends," something she called an "incredible indignation."

"I would like to receive my money back. New York State accepted my marriage as a marriage," she told CNN. "And I believe, and the Justice Department and the president agree with me, that the DOMA law is unconstitutional. DOMA is cruel. It discriminates against us for absolutely no value to the country. And we'd like to see that defeated altogether. I'd like other people not to go through what I went through."