"The rallies and marches make it clear we are not taking second-class treatment anymore and that is what drives court decisions on civil rights," he said.
The court is due to issue a ruling within 90 days of today's proceedings.
Chief Justice Ronald M. George said that the court's decision on whether Proposition 8 deprives gay citizens of an inalienable right "is going to have implications for future efforts if everything that could conceivably be characterized as an inalienable right is outside the people's initiative power."
Along with the constitutionality arguments, the court also heard opinions on whether the 18,000 same-sex marriages performed before the November election should remain legally recognized even if the justices uphold Proposition 8.
In an interview on ABC News Now, author and gay rights activist Cleve Jones admitted Proposition 8 opponents, "those who favor full equality for all citizens [and] equal protection under the law, didn't run a very effective campaign" before the November election.
"I think the opposition did a very good job of clouding the issue and trying to make it about sex or religion or children," he added.
But he said, "I don't think that we've taken a giant step backwards. I think history is on our side."
Jones added that "regardless of the court's decision, it's very important for people to understand that the majority of the rights that heterosexual people are granted through marriage will still be denied to gay people. So those who are in same-sex relationships or married, who have been married in Massachusetts and in Connecticut and even those marriages that were performed in California, they're still second class. They don't have the most important rights, which, of course, are determined by the federal government, not the states. I'm talking particularly about Social Security."