I think the difficulty in fighting this is like fighting 'green snow'.
IOW, same-sex marriage isn't really marriage, just like green snow isn't snow.
How do you fight something that doesn't really exist?
Excellent question. Now ask yourself this: If my local government decided to tax me to provide funds for plowing green snow off the streets during the months of June, July and August, would I fight it? If so, how?
Fight gay marriage the same way you'd fight green snow plowing: Point out the fiction, point out the real reasons the fiction is being implemented, and raise HEL with your elected reps.