It reached this conclusion through a lot of philosophizing about "the right of same-sex couples to have their official family relationship accorded the same dignity, respect and stature as that accorded to other officially recognized family relationships." But the state constitution (like the federal one) does not traffic in mushy terms like "dignity" and "stature." When a court puts such heavy reliance on amorphous concepts, it telegraphs that it will not be tied down by the actual words of the state charter.
For further proof, consider that while the California constitution forbids discrimination on the basis of "sex, race, creed, color, or national or ethnic origin," it does (SET ITAL) not (END ITAL) forbid discrimination on the basis of sexual orientation. The justices somehow found something in the document that the authors thought they omitted.
Prudence and caution, which are virtues in the executive and the legislative branch, are no sin in the judiciary, either. What those attributes dictated here is that the court give civil unions a fair interval to show their merits or flaws in practice, rather than rushing in to pronounce them inadequate.
The justices would have been wise to mark time while the people of California continued on their path toward full equality for gays. Instead, the court has practically exhorted them to stop the journey. Opponents of gay rights have mounted a drive to put a constitutional amendment on the ballot in November, which stands a good chance of passing.
The exercise may end up not only overturning the Supreme Court's presumptuous decree but hardening public attitudes against the whole idea for years to come. In time, Californians would probably be inclined to embrace gay marriage. But if you insist they go there today, don't be surprised if they refuse.