A federal judge who will decide the constitutionality of California’s ban on same-sex marriage wants lawyers during closing arguments Wednesday to discuss the meaning of “choice” in sexual orientation and a possible finding that Proposition 8 attempted to “enforce private morality.”

Chief U.S. District Judge Vaughn R. Walker, who has been presiding over the federal marriage trial in San Francisco, sent lawyers a list of 39 questions he wants addressed when the trial ends Wednesday after four hours of closing arguments.

Walker’s questions reveal some of the issues still troubling him and suggest that his decision will rely heavily on evidence about the nature of sexual orientation, the effect of Proposition 8, and the similarities and differences between heterosexual and same-sex couples. He asked each side 12 questions and posed an additional 15 to both.

Testimony began in January and ended 2 1/2 weeks later. Opponents of Proposition 8 called 16 witnesses and supporters two. A protracted legal fight then ensued over the kinds of internal communications the campaigns had to disclose, a squabble that helped delay the trial’s end.

Attorneys in the case suspect that Walker has by now produced a draft opinion that touches on issues he has raised in his questions.

Among the questions asked of attorneys for the Proposition 8 opponents is how much importance should be placed on the motivation of voters who approved the marriage ban. Gay rights lawyers argue that Proposition 8 stemmed from animus against gays and lesbians.

Even if a ban on gay marriage has no rational purpose, Walker asked, is it legal if voters truly believed it would prevent harm? He cited a case that said a legislature could enact a law based on a “common-sense” notion that children fare best in homes with a mother and a father.

“Do the voters’ honest beliefs in the absence of supporting evidence have any bearing on the constitutionality of Proposition 8?” Walker asked.

He also expressed interest in testimony that suggested women were more flexible in their sexual orientation than men. A survey described during testimony reported that 88% of gay men said they had no choice in their sexual orientation while 68% of lesbians felt that way.

“What are the constitutional consequences if the evidence shows that sexual orientation is immutable for men but not for women?” Walker asked. “Must gay men and lesbians be treated identically under the Equal Protection Clause?”

Walker also observed that same-sex relationships received no legal protection in the United States until “very recently.”

“How does this fact square with plaintiffs’ claim that marriage between persons of the same sex enjoys the status of a fundamental right entitled to constitutional protection?” Walker asked.

Another query revealed possible skepticism about the argument that legalizing gay marriage would reduce discrimination against gays and lesbians. He asked the lawyers for two same-sex couples to cite evidence for that contention.

Turning to the supporters of Proposition 8, Walker asked them how same-sex marriage would have negative social consequences and to describe how it would drastically change marriage as an institution.

“Why is legislating based on moral disapproval of homosexuality not tantamount to discrimination?” Walker asked both sides. “What evidence in the record shows that a belief based in morality cannot also be discriminatory?”

Walker, a Republican appointee with libertarian views, also explored the similarities between marriage and the state’s domestic partnership law.

“If, under California law, registered domestic partners are to be treated just like married spouses, what purpose is served by differentiating — in name only — between same-sex and opposite-sex unions?” he asked.

Walker has rejected a request by the media to broadcast closing arguments. They will be heard in his courtroom and televised into an overflow room in the Civic Center courthouse. His decision on cameras late last month followed a U.S. Supreme Court ruling in January that stopped him from broadcasting the trial on the Internet.

A ruling in the lawsuit brought by two couples who want to marry is expected this summer. The decision will probably be appealed to the U.S. 9th Circuit Court of Appeals and then to the U.S. Supreme Court.

Proposition 8 amended the California Constitution to reinstate a marriage ban that the state Supreme Court had lifted six months earlier. The California Supreme Court later ruled that the measure did not violate the state Constitution.

The federal lawsuit argues that Proposition 8 violates federal constitutional rights to equal protection and due process. A Los Angeles-based group funding the litigation hired conservative former U.S. Solicitor Gen. Ted Olson and noted liberal litigator David Boies — the duo who squared off in Bush vs. Gore — to represent the challengers.

Supporters of Proposition 8 argue that children fare best in families with two opposite-sex parents and that Proposition 8 simply preserved the historical and traditional definition of marriage.

maura.dolan@latimes.com