(CNN) -- Attorneys on both sides of the same-sex marriage debate faced off Monday in a federal appeals court in California, as a panel of judges heard arguments about the constitutionality of Proposition 8.





In August, a federal judge ruled that the voter-approved measure, which defined marriage as a union between a man and a woman, violated the U.S. Constitution. The Ninth U.S. Circuit Court of Appeals considered an appeal of that judge's ruling Monday.





The three-judge panel opened Monday's hearing with tough questioning of parties seeking to appeal the decision, including ProtectMarriage.com and Isabel Vargas, who's a 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 Charles Cooper, representing ProtectMarriage.com.





"Your honor, I don't have a case," Cooper responded, referring to relevant court cases.





Cooper later urged the panel to consider how the California Supreme Court earlier ruled in favor of the voter-approved Proposition 8.





"If you don't agree with me that we have standing," Cooper told the court, "then I do urge you to answer the California Supreme Court decision."





Then 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.





After a lengthy exchange, Reinhardt appeared frustrated and stated: "If you don't know the answer, say so, as the prior attorney did."





In asserting that his client has 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 arguments in San Francisco, California, were being divided into two hour-long sessions -- one over the legal standing of those appealing the decision, and one over the constitutionality of Proposition 8.





Monday's hearing is the latest in a lengthy legal 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.
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