Although gay couples are barred from marrying in 41 out of 50 US states, including some where bans were enacted by popular vote, polls show more Americans than ever -- 58 percent in one recent survey -- support the practice.
And, following the example of President Barack Obama, a number of influential politicians, economists, actors and athletes have spoken up in support of marriage equality.
Now, the country's top court has agreed to weigh in, by considering two cases: one an appeal on California's gay marriage ban and another challenging the national law that limits federal benefits to heterosexual couples.
"Whatever they decide it is going to be high-profile and it is going to get lots of attention," said Thomas Keck, political science professor at Syracuse University.
The top court will first look at a case that some experts say opens the path to legalizing gay marriage at the national level.
The case centers on California's constitutional same-sex marriage ban, voted in by a referendum on "Proposition 8," which limits marriage to one man and one woman.
In the West Coast state, where same-sex marriage was briefly allowed, gays and lesbians can now only obtain civil unions, which offer the same state benefits as marriage.
The White House, though not directly involved, has urged the court to legalize gay marriage in California as well as in the seven other states -- and soon to be a ninth, after a recent vote in Colorado -- that allow civil unions.
"It's reasonable to conclude that the states are only withholding the label of marriage for same-sex couples in an attempt to stigmatize those couples on the basis of their sexual orientation," said lawyer Elizabeth Wydra.
Wydra -- who works the Constitutional Accountability Center, which filed an amicus brief in support of gay couples -- said the US Constitution prohibits such discrimination.
And she argued that it was possible "that the court will rule categorically that the Constitution guarantees marriage equality in every state, because the Constitution protects all persons from discrimination."
In the second case, to be heard Wednesday, the nine justices will take up the federal Defense of Marriage Act, or DOMA, which since 1996 has defined marriage as a union "between one man and one woman."
Section three of the law bars any same-sex couple -- even those legally married in the nine states and the US capital that allow it -- from the rights due to heterosexual married couples, like those on inheritance.
Plaintiff Edith Windsor, a gay woman who married in Canada, had to pay $363,000 in federal estate taxes on the property she inherited when her wife died -- money she would not have had to pay if she'd married a man.
Instead of defending of the law, as it had done in the lower courts, the Obama administration has switched sides and now asks for the outright repeal of Section Three of DOMA, which it says is unconstitutional.
Former president Bill Clinton, who enacted the law while in office, and his wife Hillary, who served as secretary of state under Obama, have also called for overturning the law.
A group of Republican lawmakers from the House of Representatives is defending DOMA. It is also supported by a plethora of briefs from churches and conservative groups.
"Marriage has always been understood in US law as involving an opposite sex couple, not two men or two women," argued Paul Linton, lawyer for the Family Research Council, citing same-sex couples' inability to procreate.
The appeal to tradition is one seen as likely to convince two of the most conservative justices on the current bench, Antonin Scalia and Samuel Alito, who have often cited that kind of precedent to maintain the status quo.
But the verdict could depend on whether Chief Justice John Roberts and fellow conservative Anthony Kennedy will, as many experts expect, rule the law is contrary to the Constitution's protections of equality.