prop8triallede.jpgThe nation’s first federal trial on the constitutionality of a same-sex marriage ban has moved another step closer to completion in U.S. District Court in San Francisco.

The two sides in the case–a pair of same-sex couples on one hand and the sponsors of California’s Proposition 8 on the other–filed their final briefs late Friday shortly before a midnight deadline.

The next step will be for U.S. District Judge Vaughn Walker to schedule closing arguments in the case sometime this spring.

After that, Walker will issue a later written ruling on a lawsuit in which the two couples claim Proposition 8 violates their federal constitutional rights.

Earlier, Walker heard evidence in the case in a two and one-half week nonjury trial in January.

Proposition 8, enacted by state voters in 2008, provides that “only marriage between a man and a woman is valid or recognized in California.”

The initiative overturned a California Supreme Court ruling that held there is a state constitutional right to gay and lesbian marriage.

The lawsuit filed by Kristin Perry and Sandra Stier of Berkeley and Paul Katami and Jeffrey Zarrillo of Burbank last year took a different approach to California marriage battles and was based on federal rather than state constitutional arguments.

Among other arguments in their final brief, the two couples cite a series of U.S. Supreme Court rulings in which the high court has said “the decision to marry is a fundamental right” and “marriage is one of the basic civil rights of man.”

But Proposition 8 sponsors argued in their brief that same-sex relationships cannot be “shoehorned into the fundamental right to marry that has been recognized by the Supreme Court.”

Their lawyers wrote, “The Supreme Court has never so much as hinted that the fundamental right to marry extends beyond opposite-sex unions.”

The Proposition 8 sponsors also contend that a purpose of marriage is “promoting enduring and stable family structures for the responsible raising and care of children by their biological parents.”

Lawyers for the two couples claim, however, that evidence at the trial showed that children raised by lesbian and gay parents are as well-adjusted as those raised by heterosexual parents.

The attorneys cited the experience of the Netherlands and Massachusetts, where gay marriage is legal, and wrote, “There is no evidence that there has been any harm to the institution of marriage as a result of allowing same-couples to marry.”

The trial in Walker’s court is only the first stage in resolving the couples’ lawsuit.

Walker’s eventual ruling can be appealed to a three-judge panel of the 9th U.S. Circuit Court of Appeals in San Francisco, then to an 11-judge panel of the same court and finally to the U.S. Supreme Court.

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