By ROBERT BARNES

San Francisco – A federal judge will hear closing arguments Wednesday in a groundbreaking trial on a lawsuit that seeks to establish marriage as a fundamental constitutional right for gays and lesbians.

Chief District Judge Vaughn R. Walker has told lawyers for the two sides to be ready to answer a host of questions that go to the heart of the nation’s debate over gay rights. Is sexual orientation a choice? Does recognizing same-sex marriage threaten the institution of marriage? Were California voters justified in 2008 when they approved Proposition 8 to amend their constitution to limit marriage to a man and a woman, just as 30 other states have?

The proponents of Prop 8 told the court in papers filed Tuesday that it was not discrimination for voters to approve a version of marriage that “has been a fundamental, definitional feature of that institution throughout history at common law, in this country and, almost without exception, in every civilized society that has ever existed.”

The plaintiffs are two couples — lesbians from the Bay area and gay men from Southern California — who arerepresented by former Republican U.S. solicitor general Theodore B. Olson and his Democratic rival inBush v. Gore, David Boies.

Their Tuesday filing to Walker said their clients were not seeking a “new” constitutional right, but simply access to “an existing constitutional right that has long been denied to gay men and lesbians.

“The mere longevity of those discriminatory and irrational restrictions on the right to marry is a constitutionally inadequate ground for continuing to exclude gay men and lesbians from this vital personal right.”

The plaintiffs said Proposition 8′s restrictions violate the due process and equal protection clauses of the U.S. Constitution and serve no legitimate governmental interest.

Whatever Walker’s decision, which is expected in coming weeks, the case is headed for an appeals process — first at the historically liberal U.S. Court of Appeals for the 9th Circuit in San Francisco — that will likely end at the Supreme Court.

The justices have never squarely confronted the issue of whether gays have a constitutionally protected right to marry.

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