Amending the Constitution for something like this will only turn the Constitution into a political football, and destroy it as an arbiter of our political affairs.
The way to defend marriage is through impeachment of judges who misapply the Constitution. We'll make much further headway by doing that, and it really is the more honorable course.
Impeachment and the threat of it are an excellent and necessary correction for this mess we are in with judicial activism. However, I do not see that as the only response to the current marriage threat. I support the FMA and I hope it will be ratified. Even if it is only pushed toward the brink, it may do an enormous amount of good as a shot across the bows of the activist's ships. That might be enough.
Also, Amendments to the Constitution being political "footballs" I do not see as a problem. An active political process is good for the country.