The momentum to acknowledge gay marriage escalated dramatically in 2013, when the court found Section 3 of the 1996 Defence of Marriage Act - a law that denied federal tax, health and other benefits to gay couples by defining "spouse" in heterosexual terms - was unconstitutional. States quickly began rescinding bans on same-sex unions.
Today, gay marriage is legal in 35 states and the District of Columbia. Lower court rulings in Florida and Missouri also recognise gay couples. Rulings allowing same-sex marriages in South Dakota and Alabama have been stayed, pending appeal, so couples cannot yet marry. Over 70 per cent of Americans live in jurisdictions where same-sex marriages are legal.
"Our strategy has always been to build to a Supreme Court ruling that would bring the country to a national resolution," says Evan Wolfson, president of Freedom to Marry.
"Unlike New Zealand, marriage in the US is regulated first at the state level, and then is subject to the US constitution. There is no national marriage law.
"We've set out to win by building a critical mass of states, and a critical mass of public support, that would enable the Supreme Court to bring the discriminatory states across the threshold. And we hope that we've now built that critical mass and the court will finish the job."
Public support for same-sex marriage is now in the majority. A March 2014 Washington Post-ABC News poll found 59 per cent of Americans support gay marriage, with 34 per cent opposed. An October 2014 Pew Centre poll found 52 per cent favoured gay marriage, with opposition greatest in parts of the mostly Southern Bible Belt, America's evangelical heartland, where aggregated support for same-sex unions is 44 per cent, as against 61 per cent in the East, 58 per cent in the West and 52 per cent in the Midwest. There is also a marked generational shift.
"The younger you are the more likely you are to be comfortable with gay people and gay and lesbian rights," says Steve Sanders, who teaches constitutional law at the University of Indiana's Maurer School of Law. "The Grim Reaper delivers a certain percentage of the vote on this each year and that's been accelerating."
The US Attorney-General, Eric Holder, says the Justice Department will probably file an amicus, or "friends of the court", brief in favour of same-sex marriage. And President Barack Obama is on record as saying he hopes the court will come "to the right decision" in favouring marriage reform.
The Supreme Court will consolidate four cases from the sixth circuit - which covers Kentucky, Ohio, Tennessee and Michigan - into two questions: can states legally deny gay couples the right to marry, and can states that ban gay unions deny the legality of a marriage licence issued elsewhere.
Sanders predicts a 5:4 court decision for same-sex marriage. Theoretically, there is a slight chance of a two-step decision: first ruling that states cannot deny couples with out-of-state licences the same rights as heterosexuals; followed by a ruling affirming the constitutional right to same-sex marriage.
"But if they decide the constitutional right to marriage includes gays and lesbians, the interstate recognition of same-sex marriages becomes moot."
He says the Justice Department's amicus brief shows the federal Government wants to bring its moral authority to bear - denying same-sex unions violate civil rights - while guaranteeing economic justice to its staff.
"Most people have health insurance through their employers. It is typically the case that employees add their spouse and children on to this coverage at advantageous prices, instead of them buying policies on the open market.
"If a heterosexual couple can do that and a same-sex couple can't, that's a matter of hundreds, if not thousands, of dollars one is advantaged over another."
But what if the court decides same-sex unions are not constitutional? Would this nullify existing state marriages?
Wolfson believes such a reversal would not "roll back the clock".
But it would be a recipe for legal confusion. "It would be a mess for families, a mess for business, a mess for communities and a mess for states."
Nonetheless, as same-sex marriage moves towards America's highest court for what advocates hope will be a constitutional resolution, a conservative backlash is evident.
In Oklahoma some 60 per cent reject gay unions. Ted Cruz, the Republican senator and Tea Party darling from Texas, has vowed to introduce a US constitutional amendment to ban same-sex marriage, a cry taken up this month by Bobby Jindal, Louisiana's Republican governor. Both have 2016 presidential hopes.
"I believe that marriage is between a man and a woman," Jindal told ABC News, maybe eying his party's hardcore base as several conservative states like Tennessee, Georgia and Mississippi moved their 2016 primaries to March 1. "My Christian faith teaches me that."
In Texas, Oklahoma and South Carolina Republican lawmakers threaten to stop salaries for officials who issue marriage licences to gays - defying US courts. In Georgia opponents fear a proposed bill would allow businesses to discriminate against gays. And in Virginia, Utah and South Carolina, conservatives insist officials can refuse to conduct gay marriages on religious grounds, citing as precedent doctors who refuse to perform abortions for religious reasons.
All of which is red meat for evangelicals, especially older people, and may activate the Republican base in next year's presidential primaries. So while mainstream Republicans like Jeb Bush and Chris Christie want to move on, far right activists see gay marriage as a litmus test for candidates and want to drag the issue into the 2016 race whatever the court decides.
