Couldn't they just as easily make a law defending the churches from this action? A law such as this wouldn't just cover marriage. It would have to cover the rights of the churches themselves as well, along with any other religiously affiliated places. THey still have the right to not marry. You also have to realize gay couples will still be able to go to churches that allow same sex marriages.
If that happened, I'm not sure what else you could say against it.