Just have a federal judge say it violates the US Constitution.
Then it goes to the USSC and if they overturn it, then Gay Marriage becomes the law of the USA.
There is already binding Supreme Court precedent that explicitly says that the US Constitution does not provide a right to same-sex marriage (Baker v. Nelson). Even the 9th circuit wouldn't be crazy enough to completely ignore directly relevant precedent. The 9th circuit has reached a lot of crazy results, but I don't think even they are loony enough to reject direct SCOTUS precedent and invent a right to gay marriage. And even if they did, I'm confident that the SCOTUS would smack that idea down.