The California Supreme Court on Tuesday upheld the Proposition 8 measure that California voters approved in November, banning same-sex marriage in California. The court, however, said that the 18,000 marriages that were performed before the vote will remain legal.

The court was split 6-1 on upholding Proposition 8, but was unanimous on keeping the current marriages intact. The announcement was met with a crowd chanting "shame on you" outside the court building in San Francisco.

Lawyers for the opposition had argued that Proposition 8 alters California's Constitution, meaning that a constitutional convention would need to be held in order to make the change. They also argued that the proposition takes away fundamental rights guaranteed to same-sex couple under the U.S. Constitution. The six justices who voted against overturning the proposition didn't see it that way.

Gay rights advocates have said they will take the issue back to the ballot box as soon as next year, asking voters to repeal the decision.

The legal fight for same-sex marriage in California began in 2004 when San Francisco Mayor Gavin Newsom, against state law, began issuing marriage licenses to same-sex couples. The California Supreme Court later declared those marriages illegal, but the case went to the San Francisco Superior Court, which declared the marriage ban unconstitutional. The case eventually ended up in the state Supreme Court, which upheld the legality of same-sex marriages. Last fall, Proposition 8 reversed that ruling, making same-sex marriages illegal.

Four states currently allow same-sex marriage: Connecticut, Maine, Massachusetts, and Iowa. Vermont will begin offering marriage licenses to gay and lesbian couples in September. In New York, a bill introduced by Governor David Paterson allowing same-sex marriage has been passed by the state Assembly and now resides with the Senate.

Demonstrations are planned across the country tonight. Click here to find one in your city.