After those unions were later ruled invalid, the California Supreme Court legalized marriages for same-sex couples in 2008, but just months later voters in the state passed Proposition 8, which defined marriage between a man and a woman in the state constitution.
Two years later, a federal court ruled Proposition 8 was unconstitutional and then, in 2015, the U.S. Supreme Court legalized marriage for same-sex couples across the country.
But the language placed in the California Constitution by Proposition 8 has never been removed.
State Senator Scott Wiener worries that the U.S. Supreme Court could reverse its 2015 decision on marriage for same-sex couples much like it reversed Roe v. Wade, which had guaranteed the constitutional right to an abortion.
"In 2008, the voters put a ban on LGBT marriage in the constitution. That language is still in there. It's not enforceable today. It's a dead letter because the Supreme Court says people have a fundamental right to marry. But if the court was able to overturn its decision on same-sex marriage, then that dead-letter language would come back to life," said State Senator Scott Wiener, who co-authored Proposition 3, which would enshrine the right to marry in the state constitution.
That dead phrase in the constitution says, "only marriage between a man and a woman is valid or recognized in California."
Proposition 3 would eliminate that phrase and replace it with one that says, "The right to marry is a fundamental right."
It's a simple phrase that worries some people.
"By making marriage a fundamental right without any other definition, it really opens up Pandora's box and would allow legal avenues for all sorts of relationships to be considered," says Jonathan Keller, president and CEO of the California Family Council.
"There's no definition on the genetic relationship between individuals who can get married under Proposition 3, on the age of people who can get married, and I think most crucially, there's no definition on the number of people who can get married.
Keller points to new laws enacted this year by Oakland and Berkeley recognizing polyamorous families.
It protects multi-partner families, people in asexual relationships, as well as single parents and multi-generational households, from discrimination in housing and civil services.
"This new constitutional amendment would override existing state laws that are defining marriage. It would override a law that says polygamy was illegal. And if someone has a fundamental right to get married, nothing in this constitutional amendment or anywhere else in the constitution would say that marriage is only limited to two individuals.
Keller worries people in polygamous relationships will sue to obtain legal recognition statewide.
Wiener says the rhetoric is just a scare tactic.
"If the law says, which it does, that you cannot marry your sibling, this won't change that. If the law says that you can't marry an animal, which you can't, this will not change that," says Wiener.
Proposition 3 is personal for Oakland residents Stephisha and Viveca Ycoy-Walton.
"We live in fear for others every day that it can happen because we have seen it," says Viveca Ycoy-Walton, who married her partner in 2013 after the U.S. Supreme Court struck down a lawsuit aiming to reinstate California's ban on same-sex marriage, thus reinstating the right to same-sex marriage in the state.
"It may seem like a formality. But that formality is the difference between somebody's happy ending or their disappointment and rejection," says Viveca.
The couple is active in campaigning for Proposition 3 to ensure the right to marriage for everyone is enshrined in the state constitution.
"Formalities matter. If it didn't matter it wouldn't still be there. And if it didn't matter we wouldn't have to fight to have it removed," says Stephisha Ycoy-Walton.
Being legally married allowed them to have both their names on their son's birth certificate, which helped put him at ease when he was being picked on at school.
"Somebody told me I was adopted because I had two moms," said Karter Ycoy-Walton, who felt confused by other kids teasing.
Seeing his birth certificate with both his mothers' names on it was comforting and reassuring.