Back in 2008, the California Supreme Court briefly made same-sex marriage legal in that state. but the ruling was squashed within a month by the passage of Proposition 8, a referendum which amended the California state constitution to enforce bigotry. Now, the validity of that amendment is being challenged in federal court.
The challenge is simple: denying marriage to same-sex couples violates the Fourteenth Amendment's guarantees of due process and equal protection under the law. The logic is the same as that underlying Loving v. Virginia, the 1967 case which overturned bans on interracial marriage, and effectively asks the court to recognise that homophobia is as invidious and unjustified a moral evil as racism. The question is whether the US as a nation can recognise that, or whether it will continue to uphold Christian bigotry.