"We are confident where the American people are going with this," said Theodore Olson. "We don't know for sure what the United States Supreme Court is going to do, but we're very, very grateful they listened, they heard, they asked hard questions, and there's no denying where the right is."
Spirited rallies on both sides of debate
One of the plaintiff couples -- Paul Katami and Jeff Zarrillo of Burbank, California, who want to marry but can't because of Proposition 8 -- contend the state is discriminating against them because of their sexuality.
"This is about our freedom and our liberty," Katami said. "We are not trying to topple marriage. We are not trying to redefine marriage. What we are trying to say is that equality is the backbone of our country."
This view was echoed by fellow same-sex marriage supporters, who rallied outside the court with the hope that the justices would eventually issue a broad ruling to strike down bans nationwide.
"We are not asking for anything more than our neighbors, friends and family, but certainly expect no less," said Todd Bluntworth, who spoke with his husband and their two children.
But opponents who rallied nearby -- some of them carrying signs reading, "Kids do best with a mom & dad" -- urged the court to keep out of the issue and leave the decision to states.
"If you want to get married, go to one of the states that allows gay marriage," said Carl Boyd Jr., a conservative Nashville talk-show host. "Stop trying to force your agenda down our throats. Quit trying to bully the American people with the homosexual agenda."
Patchwork state laws
The court could historically alter how the law treats marriage, striking down laws across the country banning same-sex marriage and matching an apparent cultural shift toward acceptance of same-sex couples.
Or it could leave the current patchwork of state laws in place, choosing to let state legislatures and state courts sort it all out.
Forty-one states now forbid same-sex marriage, although nine of them allow civil partnerships. Nine other states allow same-sex marriage, and about 120,000 same-sex couples have gotten married, according to estimates.
California's ban seems to run counter to polls that show rising support overall for same-sex marriage.
A CNN/ORC International poll released on Monday found 53% of Americans now support same-sex marriage, up from 40% in 2007. As to how the federal government should handle the issue -- the subject of Wednesday's hearing -- another CNN/ORC International poll out Tuesday found 56% of the public feels the federal government should also legally recognize same-sex marriages.
The Justice Department will argue against the Defense of Marriage Act, a federal law, on Wednesday.
This comes as President Barack Obama has changed positions on same-sex marriage over his political career. Now fully supportive, he's said would strike down California's law if he sat on the Supreme Court.
Other prominent politicians have voiced timely opinions in recent weeks, indicating the matter's importance in 21st century American politics.
Hillary Clinton, the former secretary of state and possible 2016 Democratic presidential candidate, publicly backed same-sex marriage earlier this month.
The Republican party has historically backed efforts to define marriage as only between a man and a woman. But there's been notable movement of late among Republicans, including Sen. Rob Portman's recent shift to support same-sex marriage, which he made two years after his son revealed to him that he is gay.
"There is no putting this genie back in the bottle. It is undeniable. The shift is here, and we're not going back," Republican strategist and CNN contributor Ana Navarro said Sunday.