SAN FRANCISCO (CNS) -- A challenge in federal court to the constitutionality of the voter-approved 2008 California ballot measure that defined marriage as the union of one man and one woman, began Jan 11 in the San Francisco courtroom of Chief U.S. District Judge Vaughn Walker.

At the first federal trial on same-sex marriage and a possible preview of a debate that may eventually reach the U.S. Supreme Court, a lawyer challenging Proposition 8 told Walker that marriage is a basic civil right and the courts must extend it to all couples regardless of sexual orientation.

"This case is about marriage and equality," said Theodore Olson, who represents two same-sex couples challenging the 2008 ballot measure. "People are being denied their right to marry and their right to equality under the law."

But Charles Cooper, representing advocates of Proposition 8, said voters approved the measure because they wanted to preserve the institution of marriage -- "an institution of overriding cultural and social importance."

Voters approved Proposition 8 in November 2008, overturning a May 2008 state Supreme Court ruling that enlarged the definition of state-sanctioned marriage to include all couples. Proposition 8 provides that "only marriage between a man and a woman is valid and recognized in California."

The state's Supreme Court affirmed the constitutionality of the vote in May 2009 but said same-sex marriages that had taken place between May and November of 2008 were legal.

The California bishops and Catholic leaders across the country strongly supported Proposition 8, with the bishops saying that the acceptance of same-sex marriages would be a "radical change in public policy" that would "have many profound effects on our society." Such a change "discounts the biological and organic reality of marriage" and "diminishes the word 'marriage' to mean only a 'partnership' -- a purely adult contractual arrangement" with no thought given to children, they said.

The Catholic Church teaches that marriage is a permanent union between one man and one woman.

Among the witnesses expected to testify at the trial was Ned Dolejsi, executive director of the California Catholic Conference.

In his opening remarks, Olson characterized marriage as "one of the most pivotal personal rights essential to the orderly pursuit of happiness." It is crucial to intimate choice, spiritual unity and personal fulfillment, he said.

Cooper said voters were motivated by "special regard for this institution" rather than by bias on the ground of sexual orientation.

"Over 52 percent voted to restore and preserve the traditional definition of marriage as the union of a man and a woman -- a definition that has prevailed in nearly every society in recorded history since long before the advent of modern religions," he said.

"The people of California have been generous in extending rights to the state's gay and lesbian population," Cooper said. "Except for marriage, gays and lesbians in California have been immensely successful in obtaining their policy goals through the political process."

However, Olson, characterizing marriage as more personal and more fundamental than domestic partnership, argued that Proposition 8 was discriminatory in its intent and has harmed the plaintiffs.

"What the state has done is give a sanction to a formal relationship that's part of our culture and part of society," Olson said. "It has labeled a separate relationship as a unique relationship that's reserved for opposite-sex couples."

Cooper said the central purpose of marriage is to promote procreation. He said the institution is also vital because it promotes the welfare of children.

"Is this institution designed for pro-child reasons, or is it to produce companionship and personal fulfillment?" Cooper asked. "Are those purposes important enough to run risks to the accomplishment of the pro-child purposes?"

If Proposition 8 is overturned, Cooper predicted, marriage rates will decline. "The procreative purposes are diluted and marriage as a pro-child institution is weakened -- that will likely follow," he said.

Walker asked Cooper why the courts should not decide the issue.

Cooper answered that the constitutional challenge is legally unsound. "Our proposition is that the 14th Amendment does not govern this issue and does not take this issue out of the hands of the people in the democratic process," he said.

The trial is expected to run at least through Jan. 20, with expert witnesses testifying on such questions as the impact of same-sex marriage on the stability of opposite-sex marriage and whether a married mother and father promote the optimal child-rearing environment.

"It is clear that our opponents are trying to re-characterize Prop 8 -- which simply restored the age-old meaning of marriage -- as part of an agenda of hate and discrimination against gays and lesbians," Andrew Pugno, general counsel for the Proposition 8 coalition ProtectMarriage.com, wrote in a blog after the trial's opening day.

"One attorney said as much, claiming that Prop 8 promotes hate crimes against gays and lesbians, depression, homelessness, etc.," Pugno added. "These claims are preposterous, and we hope that they will be seen as such as the trial progresses."

Olson said his witnesses would challenge both major defenses of Proposition 8 -- procreation and parenting. Marriage "has never been restricted to the procreative activity of those who enter into it," he said. "It has no relationship to parenting, because same-sex couples are permitted to parent."

Copyright © 2010 Catholic News Service/U.S. Conference of Catholic Bishops