"There are gays and lesbians who are not allowed to marry. It's not right. It's simply not right," said "My Big Fat Greek Wedding" star Nia Vardalos.
"There are plenty of miserable people married. Gay people should have the same right to be just as miserable," joked actor Ian Gomez.
A ruling could make the ban on same-sex marriage unenforceable in California and eight other Western states.
In the first hour of the hearing Monday, arguments focused on whether there was any legal authority, or standing, on the part of the plaintiffs to challenge the constitutionality of Prop 8 without the governor or attorney general involved.
The three-judge panel is hearing the case because a federal judge struck down Prop. 8's ban on same-sex marriage in August. Judge Vaughn Walker wrote, "Prop. 8 both unconstitutionally burdens the exercise of the fundamental right to marry and creates an irrational classification on the basis of sexual orientation." Supporters of Prop 8 then appealed that decision.
Outgoing Gov. Arnold Schwarzenegger and Attorney General Jerry Brown, who would have been responsible for enforcing the ban, refused to challenge the ruling.
"Usually we don't allow somebody who has that sort of abstract interest, as opposed to a particularized personal interest in a matter to pursue a federal lawsuit," said legal analyst Dean Johnson.
In 2008, the state Supreme Court ruled that a ban on same-sex marriage was unconstitutional and discriminated against gays. Eighteen-thousand gay couples got married. Then voters approved Prop 8 amending the constitution and banning same-sex marriage, which led to a lawsuit challenging Prop 8's constitutionality.
In the second hour of arguments, the panel heard from both sides on the constitutionality of the gay-marriage ban.
Attorney Charles Cooper, who represents sponsors of the ban, argued the state could treat same-sex couples differently when it comes to marriage without running afoul of the Constitution because "sexual relationships between men and women naturally produce children."
"Society has no particular interest in a platonic relationship between a man and a woman no matter how close it might be, or emotional relationships between other people as well, but when the relationship becomes a sexual one, society has a considerable interest in that," Cooper told the judges. "It's vital interests are actually threatened by the possibility of an unintentional and unwanted pregnancy."
Judge Stephen Reinhardt replied: "That sounds like a good argument for prohibiting divorce. But how does it relate to having two males and two females marry each other and raise children as they can in California and form a family unit where children have a happy, healthy home?"
Before Prop. 8 was passed, an estimated 18,000 same-sex couples were married.
A decision is expected within the next few months. Many people believe that whatever decision is reached by the 9th U.S. Circuit Court of Appeals will be appealed to the U.S. Supreme Court.
The Associated Press contributed to this story.