California's Proposition 8, the state's voter-approved ban on same-sex marriage, goes before the U.S. Supreme Court today.
The justices are expected to rule on the constitutionality of the measure by the end of June, after hearing one hour of arguments this morning.
The challenge to Proposition 8, which started out as a lawsuit filed by two couples in federal court in San Francisco four years ago, is one of two marriage cases being heard by the high court this week.
In a second case, the court will hear arguments Wednesday on a New York widow's challenge to the federal Defense of Marriage Act, known as DOMA.
While the Proposition 8 dispute concerns whether an individual state can prohibit gay marriage, the DOMA case has to do with whether the U.S. government can deny federal benefits and tax advantages to couples who were legally married in their state.
Nine states and Washington, D.C., currently allow same-sex marriage.
In today's arguments, the sponsors of Proposition 8, a 2008 ballot measure, are appealing a ruling in which a federal appeals court in San Francisco last year struck down the initiative.
The sponsors contend California voters were entitled to believe that restricting marriage to male-female unions benefits society because it
"advances society's vital interest in responsible procreation and childrearing."
They also argue that states, acting through ballot measures or legislatures, have the right to define marriage within their territory.
The two lesbian and gay couples from Berkeley and Burbank who challenged the initiative say it violates their constitutional rights of equal treatment and due process.
The couples agree that "marriage is a unique, venerable and essential institution. They simply want to be part of it," their lawyers said in a brief submitted to the court.
The plaintiffs say there is no proof that same-sex marriage harms heterosexual marriage. Meanwhile, they argue, being denied the status and benefits of marriage hurts gays and lesbians and the nearly 40,000 children they are raising in California.
The court could rule on Proposition 8 in any of a number of ways. It could uphold the initiative or it could strike it down on grounds that could apply to California alone, to eight states or to all 50 states.
The court could also decide to dismiss the appeal if it concludes that the sponsors lacked the legal authority to step in to defend Proposition 8 on appeal after California officials declined to do so.