Known as Proposition 8, the ban was passed into state law in 2008 after high-profile and often controversial campaigns for and against, with 52 percent of the vote supporting denying equal marriage rights to same sex couples.
"Proposition 8 cannot survive any level of scrutiny under the Equal Protection Clause," wrote San Francisco-based federal judge Vaughan R. Walker in his just-released decision. "Excluding same-sex couples from marriage is simply not rationally related to a legitimate state interest."
Today's decision applies only to California and not to the dozens of other American states that have either constitutional bans or other prohibitions against same-sex marriage. Nor does it impact federal law, which does not recognize such unions. However supporters of marriage for same-sex couples, who include but are not limited to glbt campaigners, hope that California recognising such unions will now put legal and moral pressure on other states.
Gay communities in California have cheered the court decision and are planning celebrations in San Francisco an Los Angeles.
The striking down of the ban may not be permanent though, as it is highly likely today's court decision will be appealed to the US Supreme Court.
New Zealand allows same-sex couples to form legal Civil Unions but has retained laws defining marriage as a union of one man and one woman only.