( CNN ) -- The latest legal battle over same-sex marriage is in the hands of a panel of California federal appeals court judges . Proposition 8 , a 2008 measure approved by California voters that defined marriage as a union between a man and a woman , was the focal point of Monday 's Ninth U.S . Circuit Court of Appeals hearing . Opponents of the measure want the court to uphold a federal district judge 's August decision , which ruled that the proposition violated the equal protection clause of the U.S. Constitution . Supporters note that the proposition was approved by 52 percent of voters , and argue that it protects the `` institution of marriage . '' `` I believe it 's to preserve the institution of marriage for the purposes that it has always served , '' attorney Charles Cooper said , arguing that the key reason marriage exists in society is `` that sexual relationships between men and women naturally produce children . '' Attorney Theodore Olson , representing opponents of Proposition 8 , argued that the measure is unconstitutional and denies `` the fundamental right of ... citizens to marry . '' `` California has engraved discrimination on the basis of sex and sexual orientation into its fundamental governing charter , '' he said . Ultimately , the judges ' decision could hinge as much on who 's arguing the case as what their arguments are . The first half of Monday 's hearing focused on the legal standing of the parties appealing the federal district judge 's decision . That 's because Proposition 8 is in a `` paradoxical situation , '' CNN legal analyst Jeff Toobin said . `` The usual defendants when you challenge a law of the state are the governor and attorney general of that state . But the governor and attorney general of California , Arnold Schwarzenegger and Jerry Brown , both agree with the plaintiffs that Proposition 8 is unconstitutional , '' he said . So in their place , other groups have stepped in to defend the measure . The question , Toobin explained , is `` do they have the legal right to be there ? '' Along those lines , the three-judge panel opened Monday 's hearing with tough questioning of parties seeking to appeal the decision , including ProtectMarriage.com and Isabel Vargas , deputy clerk and deputy commissioner of civil marriages for Imperial County , California , where voters overwhelmingly approved Proposition 8 . `` What 's your best case to allow for your standing in federal court ? '' one judge asked attorney Cooper , representing ProtectMarriage.com . `` Your honor , I do n't have a case , '' Cooper responded , referring to relevant past court decisions . Cooper later urged the panel to consider the California Supreme Court 's earlier ruling in favor of Proposition 8 `` if you do n't agree with me that we have standing . '' Later Judge Stephen Reinhardt questioned attorney Robert Tyler , who represented Vargas , about how a deputy clerk , instead of the clerk , could have legal standing in the appeal . Tyler told the court that the county clerks are local officers , but they perform state functions such as civil marriages . Monday 's hearing was the latest in a long-running battle over same-sex marriage in California . The state 's high court had allowed same-sex marriage , but then the 2008 Proposition 8 voter referendum passed with 52 percent of the vote . The California Supreme Court subsequently allowed that initiative to stand , saying it represented the will of the people . Two same-sex couples filed a federal challenge , saying the law violated 14th Amendment constitutional protections of due process and equal protection . On August 4 , U.S. District Judge Vaughn Walker agreed , ruling that the voter-approved ban on same-sex marriage violated the equal protection clause of the U.S. Constitution . `` Proposition 8 fails to advance any rational basis in singling out gay men and lesbians for denial of a marriage license , '' Walker , who was appointed to the federal bench by former President George H.W . Bush , wrote in his 136-page opinion . `` Indeed , the evidence shows Proposition 8 does nothing more than enshrine in the California Constitution the notion that opposite-sex couples are superior to same-sex couples . '' Same-sex marriage is currently legal in five states and in the District of Columbia . The five states are Massachusetts , Connecticut , Vermont , Iowa and New Hampshire . Walker 's landmark ruling led to a swift federal appeal that could ultimately reach the Supreme Court . CNN 's Catherine E. Shoichet and Michael Martinez contributed to this report .
A panel of judges hears arguments over whether the measure is constitutional . Opponents of Proposition 8 say it denies same-sex couples ' rights . Supporters say it protects the institution of marriage . A federal judge 's earlier ruling says the proposition violates the Constitution .