If marriage is a sacred institution according to a religious body or other theological dogma, should the government define what is sacred by defining marriage?
Take 5 minutes of your day to watch this piece. Solid arguments based on reason and not on baseless ideology.
I've heard the "But you're trampling on our religious freedom" argument before. When I pointed out that support for anti-gay marriage amendments was trampling on my religious freedoms, I was basically told that I'm not really religious anyway, so it doesn't matter.
Frankly, I doubt that's an uncommon view among those on the right.