Former U.S. Solicitor General Theodore Olson delivered the closing argument for the two same-sex couples who sued to overturn voter-approved Proposition 8, claiming it violated their civil rights under the U.S. Constitution.
He told Chief U.S. Judge Vaughn Walker that tradition or fears of harm to heterosexual unions were legally insufficient grounds to discriminate against gay couples.
"'We have always done it that way' is a corollary to 'Because I say so.' It's not a reason," Olson said at the start of the five-hour hearing. "You can't have constitutional discrimination in public schools because you have always done it that way."
Former U.S. Justice Department lawyer Charles Cooper, who represents religious and conservative groups that sponsored the 2008 ballot measure, countered that cultures around the world, previous courts and Congress all accepted the "common sense belief that children do best when they are raised by their own mother and father."
Along with closing arguments, Walker asked attorneys from both sides to answer 39 questions about gay marriage and its potential impact on society before he delivers his verdict.
The judge heard 12 days of testimony in January, but closing arguments were delayed until Wednesday to give Walker time to review the evidence.
The plaintiffs in this controversial trial are two same sex couples who say they want to get married but can't because under voter approved Proposition 8 marriage is defined as a union only between a man and a woman.
"This case for us is about how we as Americans just want to be treated equal by our government and under the law," said plaintiff Sandy Stier.
"All we're asking the court to do is ensure that we are protected under our constitution," said plaintiff Jeff Zarrillo.
Opponents of the ban see marriage as a fundamental right for all consenting adults.
"Proposition 8 divided our state into marriage haves and have nots, and that wound needs to be healed and we should all have equal marriage rights under the law," said Stuart Gaffney, a gay marriage supporter.
Supporters of Proposition 8 maintain gay marriage would damage the institution of marriage itself. They say the ban is what the people of California wanted.
"The seven million voters in California voted for Prop 8. They made a reasonable legitimate public policy decision that the Constitution permits," said Jordan Lorence, senior counsel for the Alliance Defense Fund. "The federal judge should just defer to their decision."
Currently, same-sex couples can only legally wed in Massachusetts, Iowa, Connecticut, Vermont, New Hampshire and Washington, D.C.
Walker is being asked to overturn the 2008 ballot measure that outlawed same-sex marriages in California five months after the state Supreme Court legalized it and after an estimated 18,000 couples from around the nation had tied the knot. "Yes on 8" attorneys want the judge to rule that government agencies, courts and businesses no longer have to recognize the couples as married.
"That issue was resolved by the California Supreme Court, it's a matter of California state law and I don't think Judge Walker even has jurisdiction to interfere with that decision," said legal expert Dean Johnson.
For many, this raises issues of equal protection under the law, arguing that if one gay couple can be married, all should be allowed. But opponents maintain the will of the people must be respected.
Walker did not indicate when he might make his ruling in the trial, the first in federal court to examine if states can prohibit gays from getting married. Whatever the judge does will be almost surely be reviewed by the 9th U.S. Circuit Court of Appeals and could land before the Supreme Court.
The Associated Press contributed to this report.