"The unmistakable purpose and effect of Proposition 8," they have written, "is to stigmatize gay men and lesbians -- and them alone -- and enshrine in California's Constitution that they are unequal to everyone else, that their committed relationships are ineligible for the designation 'marriage' and that they are unworthy of that most important relation in life."
Tuesday morning, activists on both sides had gathered outside the court.
Sporting gloves and puffy jackets, pro-gay-marriage activists displayed signs saying, "Marriage is a human right not a heterosexual privilege" and "the nation is ready for marriage equality."
Joey Williamson had been outside the court for 49 hours and had at least another 24 to go, having traveled from New York City waiting to hear the arguments against the Defense of Marriage Act tomorrow.
"Obviously these are hugely historic cases," Williamson says. He saw the state Supreme Court case back home in New York so tomorrow will wrap off a long saga for him. "We thought if we could see this case all the way to the end it would be super interesting."
Michael Krzywols traveled from Rhode Island with the Faith Alliance to Preserve the Sanctity of Marriage because he thinks it's important to stand up for the values he grew up with. He said marriage is an issue that has folks riled up in his state.
"A lot of people that were for a man and a woman are changing their positions al l of a sudden and just as of yesterday a lot of the mayors are getting involved," Krzywols told ABC Tuesday.
The National Organization for Marriage organized a march outside the Supreme Court that evened out the crowd, as gay-marriage supporters had outnumbered opponents earlier Tuesday morning.
The arguments on Prop 8 came after a week and a half of snowballing support for gay marriage by public figures.
Earlier this month, Ohio Republican Sen. Rob Portman became only the second sitting GOP senator ever to endorse gay marriage, citing his gay son as a reason for his changed opinion.
Former Secretary of State Hillary Clinton endorsed gay marriage last week. This week, Democratic Sens. Claire McCaskill, Mo., and Mark Warner, Va., publicly acknowledged their support for allowing gay couples to marry.
Obama later endorsed gay marriage in a 2012 interview with ABC News' Robin Roberts.
The Prop 8 and DOMA cases come before the court as popular opinion has swung in favor of allowing gay marriage.
A recent ABC News/Washington Post poll showed massive gains for gay marriage in the last decade. In the March 7-10 survey, 58 percent of respondents said they think gay marriage should be legal, while 36 percent opposed it - a near inversion of the widespread opposition to gay marriage shown in ABC/Post polling from 2003.