I would change a few things. The fact that Mormons were forced to give up a sincerely held religious belief or face persecution is not "wise" in my opinion, but sad.
Just because you don't like the idea of Indians smoking Peote for their religion doesn't mean they don't have a right to practice it. The best argument would be that you can never out-promise the Democrats. If you say "We're federalists" and promise not to impose your traditional marriage views through law, the Dems will say you're still a bigot and will champion overturning any state law or constitution which doesn't recognize "marriage equality."
Also, since people are now cheering for polygamy, arguing that gay marriage will lead to polygamy does the left's job for it.