A federal judge in California properly ruled last week that the state's ban on same-sex marriage violates the U.S. Constitution's basic guarantees of equal protection and due process. If it stands, as it should, the decision will overturn irrational, government-backed discrimination against large numbers of Americans.

The ruling by U.S. District Judge Vaughn Walker almost surely will be appealed to the Supreme Court. Given the high court's current composition, it could face a hostile reception. But it will be hard to fault the opinion's legal scholarship or factual analysis -- or its conclusion that some Americans' "moral disapproval alone is an improper basis on which to deny rights to gay men and women."

The California prohibition on same-sex marriage was the product of a 2008 initiative that voters narrowly approved. But Judge Walker noted that a long line of legal precedents makes clear that "fundamental rights may not be submitted to a vote."

During a trial earlier this year that preceded the ruling, defenders of the ban argued that the purpose of marriage is "responsible procreation," and thus gays and lesbians cannot qualify. Does that mean, opponents sensibly asked, that marriages of heterosexual couples who cannot or do not want to have children should also be prohibited by law? For that matter, should irresponsible straight parents be deprived of their rights in marriage?

To the contrary, the judge said in his ruling, witnesses failed to make a credible case that allowing gay and lesbian couples to wed weakens American society, erodes traditional civil marriage as an institution or harms heterosexual married couples. The ban, he said, merely enshrines "the notion that opposite-sex couples are superior to same-sex couples," when there is no evidence that the latter are any less capable of sustaining strong, stable marriages or raising children well.

Judge Walker's ruling observes that definitions of marriage, social roles and child-rearing standards have evolved for straight as well as gay couples. If there ever was a time when the state could justify discrimination against gay Americans who wanted to marry, he said, "that time has passed."

Domestic partnerships have been a transitional first step to give homosexual couples access to government benefits and recognition. But they still ultimately amount to second-class citizenship.

Judge Walker's calm, eloquent ruling stands in stark contrast to the sneering slogans, invented facts and fear-mongering that too often have dominated political debate over gay rights, to this day. With support from higher courts, the judge's thoughtful, evidence-based approach will supplant prejudice and demagogy.

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