The state attorney's office asked the court to invalidate the measure on the ground that certain fundamental rights, including the right to marry, are inalienable, and cannot be put up for a popular vote.
Mr. Starr, the former US independent counsel who headed an investigation that led to former President Bill Clinton's impeachment, argued that Proposition 8 doesn't revise the constitution or allow a majority to take rights away from same-sex couples because it leaves intact California's domestic partner laws.
Californians "will no doubt continue to debate the issue in terms of inalienable rights, justice, tradition and social welfare," Starr said. However, "the judiciary no longer has a role in determining the definition of marriage."
The justices challenged both sides.
"Proposition 8 only deprives same-sex couples of the nomenclature," said Associate Justice Kathryn Werdegard. "Why is that a revision rather than a mere amendment?"
The initiative establishes that a majority of voters can take away rights of a minority, responded Shannon Minter, legal director of the National Center for Lesbian Rights. "The nomenclature is not the point," Minter said. "The point is equality."
The court has to make its decision within 90 days, by June 3. By virtue of having called for oral arguments, the court likely has already formulated what is known as a "draft opinion."
"The justices might have to break new ground in interpreting the state constitution in order to classify Prop. 8 as either a minor amendment which can be adopted using the initiative process or a more profound revision … which cannot," said USC's Professor Cruz after watching the hearings.