Walker pressed on, asking again for specific "adverse consequences" that could follow expanding marriage to include same-sex couples. Cooper cited a study from the Netherlands, where gay marriage is legal, showing that straight couples were increasingly opting to become domestic partners instead of getting married.

"Has that been harmful to children in the Netherlands? What is the adverse effect?" Walker asked.

Cooper said he did not have the facts at hand.

"But it is not self-evident that there is no chance of any harm, and the people of California are entitled not to take the risk," he said.

"Since when do Constitutional rights rest on the proof of no harm?" Walker parried, adding the First Amendment right to free speech protects activities that many find offensive, "but we tolerate those in a free society."