Suppose Prop 8 passes and the California Constitution is amended
 

I'm not so sure the fight would be over.

I'm not a constitutional law expert; although I am involved in a few cases involving constitutional issues, I certainly have little background on conflict of laws issues.

But as I understand it, state constitutions cannot abridge rights which are protected by the U.S. Constitution.

Suppose the amendment passes and the California state constitution is amended to prohibit same-sex marriage; wouldn't the amendment be challengeable in the U.S. Supreme Court under the same principles as set forth by the California Supreme Court? In other words, the equal protection principles exist in the U.S. Constitution, as well as the state constitution.

Of course, with Scalia, Thomas and Alito on the U.S. Supreme Court, there's a good chance they'd find a way to allow the amendment. But I'd have bet money that Chief Justice George would not have written an opinion allowing gay marriage.