To be clear, the CA Court didn’t “uphold” Prop. 8. Let’s not forget the nature of this suit. The claim by the plaintiffs was that Prop. 8 was so wide-sweeping in its alleged eliminating of the right to same-sex “marriage” that it was actually a constitutional REVISION, instead of a mere amendment, thus it required approval by a 67% supermajority of the state Legislature (which, of course, is for gay “marriage”).
In other words, gay activists wanted to make California like Iowa and the NE states, where the people can't get an amendment on the ballot, without going through a left-leaning Legislature.
All the CA court said was that Prop. 8 was simply what its proponents intended it to be: a constitutional AMENDMENT. Therefore, the electorate DOES NOT have to get Legislative approval to place it on the ballot. Once there, all it takes is a simple majority, as Drkaje stated, to pass it.
If folks in CA are grieving about the constitution being too easy to amend, then they need to make the appropriate adjustments. Florida had a similar issue; and in 2006, the voters passed a law (against which I voted) which requires all future amendments to be passed by a 60% supermajority of the electorate.
Of course, Florida's marriage amendment (Amendment 2) still passed 62-38, a squeaker by the new standard.