City Attorney Dennis Herrera argued in a brief filed in federal court in San Francisco late Thursday that the marriage ban enacted by state voters in November as Proposition 8 has no constitutionally legitimate purpose.
Herrera wrote that the goal of Proposition 8 was to express "moral disapproval" of homosexuals and said that such a purpose is discriminatory.
The city attorney wrote, "Feelings of antipathy or discomfort toward a group of people - no matter how deeply felt or widely held - are not a legitimate purpose for singling that group out for unequal treatment by the law."
The city's argument was submitted as a friend-of-the-court brief in a lawsuit filed last month by a lesbian couple from Berkeley and a gay couple from Burbank.
Chief U.S. District Judge Vaughn Walker is scheduled to hold a hearing in San Francisco on July 2 on the couples' bid for a preliminary injunction that would reinstate gay marriage in California.
The federal lawsuit was filed the day after the California Supreme Court upheld Proposition 8 as being within the initiative power of voters.
Proposition 8 overturned a 2008 decision in which the state high court said the California Constitution provides a right to same-sex marriage.
Until now, legal battles during the past five years over same-sex marriage in California have centered on the provisions of the state constitution.
But the new lawsuit uses a different tactic and argues that Proposition 8 violated the federal constitutional guarantees of due process and equal treatment.
The two couples are represented by prominent attorneys Theodore Olson of Washington, D.C., and David Boies of Armonk, N.Y., who argued on opposite sides of the Bush v. Gore case that decided the 2000 presidential election.
On the other side, sponsors of Proposition 8 have filed a brief with Walker arguing that "nothing in the Constitution requires such a radical redefinition of the ancient institution of marriage."