"Only marriage between a man and a woman is valid or recognized in California."
An identical proposition passed in 2000 but was ruled "unconstitutional" by the CA State Supreme Court because it discriminates against a segment of the population.
If it passes again, the court is likely to make the same ruling, just as the US Supreme Court has repeatedly ruled anti-flag-burning legislation as "unconstitutional."
I don't know what percentage of the California population is gay, but nonetheless, such a law would only legalize active discrimination, just as the "separate but equal" laws did years ago. What I don't understand is why the supporters of this proposition are so adamant to try to legislate discrimination which, if passed, will be overturned yet again.