(CBSNews.com) - The Supreme Court has the opportunity to issue a landmark ruling this year as it considers the constitutionality of Proposition 8, California's ban on same-sex marriage.
It could define same-sex marriage as a constitutionally-protected right, or it could uphold the ban, setting back the gay rights movement for years. The case, however, isn't as simple as deciding whether or not same-sex couples have a right to get married.
After the Supreme Court today hears the oral arguments in the caseHollingsworth v. Perry, it has five options to choose from. Below is a rundown of the court's choices -- whether to protect the right to same-sex marriage in 50 states, zero states, nine states or one state. There's one last option -- to punt on the decision and dismiss the case.
Same-sex marriage constitutionally protected, nationwide
In the most dramatic ruling it could deliver, the Supreme Court could useHollingsworth v. Perry to rule that marriage is a constitutional right available to all Americans, gay or straight.
"The Prop. 8 case has the potential to be the Brown v. Board of Education for gay rights," UCLA School of Law Prof. Adam Winkler told CBSNews.com.
"The question is whether the court's ready to do that," said Walter Dellinger, a partner at O'Melveny & Myers who served as U.S. Solicitor General during the Clinton administration.
Dellinger noted that such a ruling would be a much larger leap than the court has taken in the past in comparable cases. For instance, when the court issued the 1954 Brown v. Board ruling, there were 17 states that mandated segregation in schools. In its 2003 Lawrence v. Texas decision, the court invalidated sodomy laws in 13 states. By contrast, there are currently 39 states with constitutional or statutory bans on same-sex marriage.
"There's reason to think there may not be five votes" for that kind of ruling, Dellinger said.
While proponents of same-sex marriage -- traditionally aligned with Democrats -- want the court to make this move, Dellinger said, "Ironically, politically nothing would be better for the Republican Party than for the court to go all the way, which would take the issue out of politics."
The issue has turned from a wedge issue for Democrats into a growing problem for Republicans as public support for same-sex marriage builds.
Same-sex marriage is not constitutionally protected
The second big move the court could make would be to declare that same-sex marriage is not protected by the Constitution.
Dellinger contends such a ruling could set the gay rights movement back as much as 25 years.
"You would then have a precedent that was a roadblock unless it was overturned," he explained. "It wouldn't stop the political movement, but the political movement is going to have some difficulty if the court rules against gay marriage because there are 30 states in one way or another that have entrenched a gay marriage ban into their constitutions, making it very hard to change the law."
Stuart Gaffney, media director for the group Marriage Equality USA, couldn't say specifically how his organization would respond to such a ruling but noted that in polls nationwide -- and in California especially -- the public is increasingly on their side. "We would certainly be keeping that in mind if we didn't prevail at the high court," he said.
Activists on both sides of the aisle, however, have noted the influence of the court.
"The Supreme Court decisions are not without effect in terms of the law but also in terms of legitimizing positions," Chris Gacek, a senior fellow at the conservative Family Research Council, told CBSNews.com. "It's like saying you have the weight of the Constitution behind you."
Gacek and other conservatives have argued the court shouldn't interfere on an issue decided directly by the voters, like Proposition 8.
No difference between marriage and civil unions
In a third alternative, the Supreme Court may decide that states that recognize same-sex civil unions or domestic partnerships -- and provide them with nearly the same benefits of marriage -- cannot legitimately bar same-sex marriage.
That decision could extend same-sex marriage to California and eight other states that currently have comprehensive civil union or domestic partnership laws -- and it's the argument the Obama administration has put forward.
Winkler said it's a "sensible argument" for the administration to argue because the government must justify its laws by pointing to valid public policy motivations.
"It's hard to know what government interests are served by denying gay people marriage when you allow them the rights of marriage," Winkler explained. "The only thing those civil union states do is deny them the right to say they're married -- what government interest could there be in that?"
At the same time, Winkler said it could be a "dangerous" argument from the perspective of gay rights proponents in one sense -- "It could discourage other states from granting civil unions," he explained.
Same-sex marriage is still a hotly-debated issue on the state level, of course -- Colorado just this month legalized civil unions, after a years-long debate on the issue. In 2006, the state passed a constitutional ban on same-sex marriage.
"Think of what will play out the next time a state like Colorado wants to extend civil unions but not marriage," Winkler said. If the court takes this third option, "there won't be that middle ground available."
Same-sex marriage protected in California
In a fourth option, the Supreme Court ruling could apply just to California because of the case's unique circumstances.
Voters in California passed Proposition 8, the ballot initiative banning same-sex marriage, in 2008 -- after the California Supreme Court had granted same-sex couples the right to marry. That put California voters in the unique position of taking away rights granted by the court. After Prop. 8 passed, a federal court followed by the Ninth Circuit Court of Appeals said Prop. 8 was unconstitutional.
While this option wouldn't extend same-sex marriage across the country, gay rights activists would still consider it a victory.
"Obviously, we believe that every American is entitled to the most basic freedom to marry the person they love," Gaffney said. "While we're very hopeful for a landmark ruling, we're also aware there are very specific circumstances in California. Proposition 8 divided California into marriage haves and have-nots... That is a wrong that needs to be undone."
Gaffney said that he and his husband, legally married in California, "experienced a feeling no American should have to go through... People's marriages were put up for a popular vote."
Given California's large population, a ruling in favor of marriage equality there would "truly tip the balance for the freedom to marry nationwide," he added.
Dismiss the case on procedural grounds
In yet another option, the court could decide the proponents of Proposition 8 have no standing in court and dismiss the case entirely.
Normally, California's governor and attorney general would defend a state law in court. In this case, however, they declined to appeal the district court ruling striking down Proposition 8. Instead, supporters of Proposition 8 took it upon themselves to appeal.
Dellinger filed an amicus brief with the Supreme Court arguing that the Proposition 8 supporters have no legitimate reason to be in court.
"In my view, the California case was over when the two couples won their victory in the trial court, which held that they were entitled to marriage licenses," Dellinger said. "The Supreme Court only hears cases when there are parties who have an adverse interest coming before the court... The people who sponsored the referendum do not, in my view, have a dog in that fight."
Some gay rights advocates agree with that argument.
"To some observers it could feel like a technicality, but I believe the standing issue speaks to a fundamental point of law and principle of marriage equality," Gaffney said. "You have to have been harmed to continue a lawsuit. Had the other side been harmed, that would have entitled them to make an appeal in this case. Marriage equality means the world to same-sex couples and their families and loved ones, and it doesn't take away from anyone else."
If the Supreme Court agrees that the Proposition 8 supporters have no standing, it's not exactly clear at this point what the practical impact would be, Dellinger said. "The effect in California, whether it applies statewide and everybody in the state can get marriage licenses, or whether it applies to these two couples or two counties, all that is to be resolved subsequent to the decision in this case," he said.