The bigger crowd, numbering a few thousand, rallied boisterously in support of marriage equality, turning up in colorful, placard-waving hordes as the sun rose on an otherwise chilly Washington morning.
A parallel crowd of conservative and religious campaigners was a no less striking sight, with bagpipers and patriotic red, white and blue paraphernalia.
Predominantly white, young and professional, the pro-equality group included not just gay men and lesbians, but also many heterosexual allies of what some call the final act in the civil rights struggle.
Among them was Melissa Wasser, 20, a pre-law college student from Ohio with a black father, a white mother and a handmade poster in her arms that read: "My parent's marriage used to be illegal, too."
"I wanted to come here and prove a point," said Wasser. She recalled with bitterness how a gay friend had endured bullying in school, yet voiced confidence that ultimately "marriage equality will spread across the country."
Nearby, a transvestite in a pink fishnet T-shirt, rainbow-colored tutu and matching high heels, with devil's horns on his head and a crucifix in his hand, gleefully danced to Lady Gaga's gay anthem "Born This Way."
In the long orderly queue for spectators going into the court, "When Harry Met Sally" director turned marriage equality activist Rob Reiner, in a crisp dark suit, proudly brandished his red admission card that bore the number one.
"We feel this is not a matter of if, but rather a matter of when," said Reiner, 66, whom some remember as bigoted Archie Bunker's long-suffering liberal son-in-law Meathead in the hit 1970s sitcom "All in the Family."
John Lewis, 54, and Stuart Gaffney, 50, from San Francisco, turned up in the bow ties and tuxedos in which they married in 2008.
They came with a sense of deja vu: they had been plaintiffs in the original lawsuit that legalized gay marriage in California before it was upended by Proposition Eight, one of the issues now facing the Supreme Court justices.
"It seems all roads lead to the Supreme Court," said Gaffney, whose own parents' marriage was once deemed illegal in the state of Missouri because one was white and the other ethnic Chinese.
Then Captain America arrived, or more precisely a stout middle-aged man costumed as the comic book hero, bearing a huge American flag and the shoulder flash of a patriotic outfit called The Guardians of the Republic.
He turned up with the smaller but no less passionate procession of opponents to gay marriage who marched on the Supreme Court from the National Mall, with a motorcycle police escort to secure their arrival.
Numbering 700 to 800, they were older, less urban and more working class. Many carried signs that read "Kids do best with a mom and dad." There was a notable turnout of Latinos and Chinese immigrants -- and many baffled looks.
"Viva Christo rey!" (long live Christ the king) shouted Cesar Franco at the head of a drum and bagpipe ensemble from the non-profit American Society for the Defense of Tradition, Family and Property from small-town Pennsylvania.
Franco, stoic as he held a US flag high, was at a loss for words when asked what he made of the pro-gay crowd surrounding him and chanting: "What do we want? Equality! When do we want it? Now!"
"I'm glad to be here," he finally acknowledged, "I'll tell you that much."
From Boston, Franciscan clergyman James Wartman echoed the Vatican line: "I want to tell the Supreme Court that if they change our understanding of what is a family, that will be a detrimental thing for the health of our nation."