In California's 2000 primary election in March of 2000, Proposition 22 (also known as the California Defense of Marriage Act) was adopted by a vote of 61.4% to 38%, in favor of amending the Family Code of California statutes. Section 308.5 explicitly defines marriage as the union of a man and a woman, and as the only valid form of marriage in the State of California.

The Mayor of San Francisco authorized city officials to issue marriage licenses from February 12 to March 11, 2004 to a number of same-sex couples. Various lawsuits ensued, and eventually, several individual suits found their way to the California Supreme Court, which ruled in re Marriage Cases that the prohibition against same-sex marriage was unconstitutional, and declared that same-sex couples had a constitutional right to marry. The justices' decision on May 15, 2008 was a four-to-three decision. The opinion, written by Chief Justice Ronald M. George, cites the California Supreme Court's 1948 decision in Perez v. Sharp, which held that California's interracial marriage ban was unconstitutional in that the ban violated "equal respect and dignity"; it further argues that marriage is a "basic civil right" that cannot be withheld from same-sex couples. Justice Ronald M. George asserts that sexual orientation is a protected class like race and gender, and that any classification or discrimination on the basis of sexual orientation is subject to strict scrutiny under the Equal Protection Clause of the California State Constitution.

At the same time, another California ballot initiative was gathering signatures in order to be placed on the ballot. Proposition 8 (or the California Marriage Protection Act) was a ballot proposition to amend the California state constitution. State Attorney General Jerry Brown changed the initial title of Proposition 8 from "Limit on Marriage," to "Eliminates the Right of Same-Sex Couples to Marry." Backers of proposition who opposed same-sex marriage immediately sued, and on August 8, California Supreme Court Judge Timothy Frawley ruled that "The attorney general did not abuse his discretion in concluding that the chief purpose and effect of the initiative is to eliminate the right of same-sex couples to marry." The amendment passed in the November 2008 California election and added Section 7.5 to the Declaration of Rights of the California Constitution. Section 7.5 stipulates that

SEC. 7.5. Only marriage between a man and a woman is valid or recognized in California.

The campaigns for and against Proposition 8 raised $39.9 million and $43.3 million, with large amounts of money coming from out of state donations.

Almost immediately, several couples sued contesting that Proposition 8 violated constitutional rights, and was a revision rather than an amendment to the California state constitution. Three of the lawsuits were consolidated as Strauss v. Horton 46 Cal.4th 364 and brought before the Supreme Court of California to be heard together in San Francisco on March 5, 2009 before Justice Kathryn Mickle Werdegar. Judge Wedegar's ruling established that Proposition 8 was valid as voted, and was a lawful amendment to the California state constitution, but that marriages performed before it went into effect would remain valid.

The American Foundation for Equal Rights (AFER) filed suit in the U.S. District Court for the Northern District of California challenging the validity and legality of Proposition 8 at the federal level. Judge Vaughn R. Walker ordered a trial known as Perry v. Schwarzenegger, which began in January 2010. Testimony addressed a variety of issues including whether being gay diminishes individuals' and couples' contribution to society, affects the ability to raise children, impairs judgment, or constitutes a mental disorder

On August 4, 2010, United States district court Judge Vaughn Walker overturned Proposition 8 in Perry v. Schwarzenegger. Judge Walker asserted that Proposition 8 violates both the Due Process and Equal Protection Clauses of the 14th Amendment to the United States Constitution. Judge Walker issued an injunction against enforcing Proposition 8 and a stay to determine suspension of his ruling pending appeal.