"The marital relationship is fundamental to the existence and survival of the race," Cooper said in closing arguments before a packed San Francisco courtroom. The reason the state regulates marriage, he said, is to steer "procreative sexual relationships" into a stable family environment so that children can be raised by their biological parents.

Other federal courts have relied on that argument to uphold measures such as Prop. 8, a November 2008 initiative that prohibited same-sex marriages in California less than six months after the state Supreme Court had legalized them.