Some folks argue that California's provision against gay marriage is a matter of state's rights. That California has a right to uphold its Constitution, as declared by its citizens. Those folks are wrong. When we have gay marriage in all 50 states - and we will - it'll be precisely because it is unConstitutional for states to “deny to any person within its jurisdiction the equal protection of the laws.” That and the Declaration of Independence proclaimed each person's right to the "pursuit of happiness." Individual states have no say in what amounts to a human rights matter - nor should they. See slavery, segregation, suffrage, etc. The U.S. Constitution supercedes California's. That may not be recognized now, but I believe it will be in the future.
Further, contrary to what many folks say, allowing for gay marriage is not encroaching on religious territory; it is not trampling on line between church and state (which oddly some of the same folks aren't usually particularly concerned with). In fact, if you read about the origins of marriage, they were not religious. Marriage simply acquired religious accoutrements over the ages. Marriages began as a civil and legal affair, not as a religious affair and it essentially involved the the acquisition of women as property. Hardly admirable. Marriage has continued to evolve, however, into a more enlightened arrangement and the addition of gay marriage to the tradition is only another step in the right direction.
Read about the history of marriage in Wikipedia, where it's currently described as "a social union or legal contract between individuals that creates kinship." No mention of religion in defining marriage there.
Or try Merriam Webster if you prefer: "the state of being united to a person of the opposite sex as husband or wife in a consensual and contractual relationship recognized by law." Again, no mention of religion. Whatsoever.
See also, About.com on the history of marriage.