The United States Supreme Court will hear argument in two cases about marriage for same-sex couples later this month. In the meantime, advocates continue to debate whether a decision recognising marriage rights will mark the end to a damaging inequality or a step in the nation's moral decline. But in both cases, a broad cross-section of the public has already staked out its position, pointedly telling the high court that the tide has turned - toward equality.

One of the cases before the court involves Edie Windsor, an 83-year-old widow who, after the death of her spouse and partner of 40 years, Thea Speyer, faces a more than $360,000 estate tax bill from the federal government. The reason for the huge bill? The Defense of Marriage Act (DOMA) blocks the US from recognising marriages of same-sex couples, even when those marriages are recognised by the taxpayer's home state. Had Edie been married to a man, instead of to a woman, her tax bill would have been $0.

In the other case, two same-sex couples challenge a measure known as Proposition 8, which was passed by California voters in 2008 to amend the state constitution to take away marriage rights from same-sex couples. The two men in the case have been together for nine years; the women - who are raising four sons - have been together for 13 years.

Despite their distinct legal issues, both cases together show how far the nation has come in recognising the equality of gay people and how intolerable a decision upholding marriage discrimination would be.

How is it possible to say this before the court has heard argument, much less issued a decision in either case? The answer lies in the dozens of friend-of-the-court (amicus) briefs that urge the justices to put an end to marriage discrimination.

Amicus briefs enable individuals and groups who are not directly part of a case to make their views known to the court and, more generally, to commit themselves publicly to one side of the debate.

In the marriage cases, this commitment weighs heavily in favour of ending marriage discrimination against same-sex couples. Consider, first, that the US House of Representatives has stepped into Edie Windsor's lawsuit as the primary defender of DOMA. Yet 172 members of the 435-member House - nearly 40 percent - have signed a brief maintaining that DOMA unconstitutionally deprives same-sex couples of the right to be treated equally.

DOMA "is not the rational result of impartial lawmaking", argue the brief's signatories, who in some cases voted for DOMA back in 1996. Forty senators joined the brief as well while only 10 senators joined a separate brief in DOMA's defence. (On a separate but related note, former President Bill Clinton, who signed DOMA into law, recently announced his view that DOMA should be struck down.)

US gay couples continue to face challenges

And that view crosses the political divide. Well over 100 prominent Republicans and self-identified conservatives also filed a brief renouncing the discrimination wrought by Proposition 8's denial of marriage rights to same-sex couples. This gathering of Republicans against DOMA reinforces, for both the court's more conservative members and the general public, the increasingly widespread view that marriage equality is both reasonable and desirable.

After all, it was the Republican Party that made DOMA a centrepiece of its political platform in the 1996 election year and has continued, until very recently, to object strenuously to marriage equality for same-sex couples. Powerfully, the Republicans' brief acknowledges that some of its signers "like many Americans have re-examined the evidence and their own positions and have concluded that there is no legitimate, fact-based reason for denying same-sex couples the same recognition in law that is available to opposite-sex couples".

In the private sector, too, hundreds of employers - from Citigroup to Nike to Marriot International - signed their own brief in support of equal marriage rights for their employees. For this group, marriage discrimination is both an affront to their employees and a burden for them, as they are forced to treat married employees differently, depending on whether those employees have a same-sex or different-sex spouse.

And these briefs are only the tip of the iceberg. Faith-based organisations, professional football players, military leaders, historians, psychologists and many more have, in an array of submissions, expressed their view that marriage discrimination is harmful not only to the same-sex couples who suffer personally when their relationships are not recognised, but also to the safety and well-being - indeed the very fabric - of American society. As the briefs filed by the Obama administration argue, there is no acceptable explanation for a law that singles out same-sex couples to ban recognition of their marriages.

The explanation offered in amicus briefs on the other side of the cases - filed by few major leaders and even fewer expert organisations - is that tradition mandates that marriage should be reserved to those who can "procreate naturally".

But American history is full of discriminatory traditions that have been brought to an end, the way so much of the American public believes marriage inequality should be. And few take seriously the argument that marriage should be reserved only to couples who have biological children - or that marriage exists only to reign in the consequences of accidental procreation.

Of course, the friend-of-the-court briefs will not decide the marriage cases; the high court's nine justices will. But what the briefs show is that many in the American public have already made their decisions and that they have come down strongly on the side of equality.

Suzanne B Goldberg is Herbert and Doris Wechsler Clinical Professor of Law; Director, Center for Gender and Sexuality Law, Columbia Law School.