Proposition 8, Olson argued, "walls off gays and lesbians from marriage, the most important relation in life."
Attorney Charles Cooper, the former Reagan administration official arguing in support of Proposition 8, stressed that recognizing same-sex marriages would "sever (marriage's) abiding connection with its historic traditional procreative purposes."
"Marriage itself is the institution that society has always used to regulate these heterosexual, procreative relationships," Cooper said.
In turn, Justice Elena Kagan countered with the example of older couples who marry despite being past child-rearing age.
"There are lots of people that get married that can't have children," Justice Stephen Breyer added.
The justices divided their time between discussing whether Proposition 8 supporters had the "standing" to argue the case, since California officials refused to defend the initiative, and the measure's underlying merits. The standing question might become an off-ramp for the case, short of a big decision. If the court decides that the Proposition 8 supporters lack standing, that kicks the case all the way back to the original decision by U.S. District Judge Vaughn Walker striking down the ballot measure. The legal consequences of that remain uncertain.
"If the issue is letting the states experiment and letting the society have more time to figure out its direction, why is taking a case now the answer?" Sotomayor asked rhetorically.
Most of the oral argument time, though, was spent on the merits.
San Francisco officials legalized gay marriage in February 2004. Several thousand same-sex couples were married before the state Supreme Court blocked the city's action. The California Supreme Court subsequently recognized same-sex marriage rights in May 2008, after which about 18,000 same-sex couples were married.
In November 2008, the state's voters approved Proposition 8, which amended the California Constitution to declare that "only marriage between a man and a woman is valid or recognized" in the state. The 9th U.S. Circuit Court of Appeals, in turn, struck down Proposition 8 on the grounds that it stripped California residents, without a legitimate justification, of a right that had previously been granted.
More than 110 reporters spilled over from the courtroom into an adjacent hallway, many sitting behind pillars that made it impossible to see all the justices who were speaking. All told, some 400 lawyers, guests and members of the public attended, with some public visitors staying for only about five minutes before being rotated out.
Justice Clarence Thomas, as is his custom, was the only high court member not to speak or ask a question.
A decision in the Proposition 8 case, as well as the Defense of Marriage Act case being heard Wednesday, is expected by the end of June.