Here are the decisions the court could reach as outlined by Georgetown University law Professor Marty Lederman writing in Bloomberg's SCOTUS (Supreme Court of the United States) Blog, with further analysis by UC Davis law Professor Vik Amar:
1) Bans on same-sex marriage are constitutional. If the Supreme Court makes this ruling, bans on same-sex marriage can continue in the 37 or 41 states (depending on how you count them) where they are now in force, including California. Individual states would still have the option to legalize same-sex marriage. Public opinion has shifted in favor of same-sex marriage around the country, and particularly in California. So it's likely that advocates would soon mount initiatives to change the laws.
2) Bans on same-sex marriage are unconstitutional. This ruling would mean that the 14th Amendment of the U.S. Constitution prohibits discrimination against same-sex couples. Every state in the union would have to grant marriage to same-sex couples.
3) States cannot deny the label of marriage once the privileges of marriage are already granted. Some states, including California, already have granted the benefits of marriage to same-sex couples through domestic partner or similar legislation. But these states still won't officially recognize same-sex couples as married.
The court could reason that these states already have accepted the legitimacy of same-sex unions, so denying these couples the status of marriage has no purpose except stigmatization. Such stigmatization is unconstitutional. Those states with bans on same-sex marriage and no benefits for same-sex couples could keep these laws in place.
Exactly how many states would be affected is not clear. Lederman counts California, Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon and Rhode Island. But Amar said California's law might have gone farther in granting privileges, so it could take further litigation to sort out exactly which states were affected.
"I personally don't expect the court to go that route," Amar said. "It punishes states that move toward equality. And it doesn't jibe with the court's other equality rulings."
4) A state that grants same-sex couples all the benefits of marriage and also has previously granted them the right to the official status of marriage can't revoke that right. This was the Ninth Circuit Court of Appeals' decision on the Prop. 8 case in 2012.
California granted same-sex couples the right to marry in May 2008 when the state Supreme Court struck down Proposition 22, an initiative in 2000 that banned same-sex marriage. In November 2008, Prop. 8 passed, trumping the California Supreme Court decision by putting the ban in the state Constitution. So, same-sex marriage was legal in California for half a year. California is the only state where the privilege existed before being revoked, so the decision would affect only California.
"That's an unlikely basis for the U.S. Supreme Court to rule on because it would discourage states from experimenting with equality," Amar said.
5) The people defending Prop. 8 before the court don't have the right to defend it. Normally the governor and attorney general of a state would defend a state law before the U.S. Supreme Court. But in this case, California Gov. Jerry Brown and Attorney General Kamala Harris have declined.
Instead, sponsors of the initiative hired their own lawyers to defend it. They claim California law grants them such standing, but there are good arguments on both sides, wrote Lederman.
If the court denies the Prop. 8 defenders' standing, then the Ninth Circuit Court of Appeals' decision would be voided, and the case would revert back to the U.S. District Court for the Northern District of California, which struck down Prop. 8.
The decision would be a default ruling against the state because it declined to defend Prop. 8 -- sort of like a forfeit when one soccer team doesn't show up for a scheduled match. It might change only the status of the two couples who filed the original lawsuit, Kristin Perry and Sandra Stier of Berkeley and Paul Katami and Jeffrey Zarrillo of Los Angeles. It is possible that only they would be allowed to marry, but no one else would, said Amar.
Under that scenario, same-sex marriage advocates would have to file a new class-action lawsuit in which all same-sex couples are plaintiffs, he said. He estimates this would likely proceed through the court in a matter of months, extending the district court's ruling that Prop. 8 is unconstitutional to all same-sex couples in the country.
The plaintiffs themselves have argued that other same-sex couples should gain the right to marriage even if the court ruled in this way, because the two couples would be harmed by being "stigmatized" if they are the only same-sex couples who can marry, wrote Lederman.
A final wild card is the court's separate ruling on the Defense of Marriage Act. "If they reject the DOMA," said Amar, "then — depending on the reasons they give — it's going to be hard to say that states can continue to deny rights to same-sex marriage."
In the past, the court has generally ruled that federal law trumps state law. For example, the federal government is continuing to enforce laws against marijuana use, even though some states now allow it.
It's still possible, though, that the high court will break with this precedent and strike down the DOMA but decide to defer to each state on its same-sex marriage laws. This would leave bans on same-sex marriage intact. This would mean that married same-sex couples could qualify for benefits under federal law. But it would not allow same-sex couples to get married in states that prohibit these marriages.