Remember the Law of Merited Impossibility: It will never happen, and when it does, you bigots will deserve it.
And no doubt members of any faith which still accepts polygamy won’t oppose its legalization.
(As for other non-Western religionists: did they vocally join in opposing SSM? No. ‘Nuff said.)
Will trad members of The Tribe stick their neck out and join us in opposing it? As with the aforementioned commenter (and as he pointed out), their ancestors used to practice it; I doubt they care that much. And with their more liberal brethren like the author above making the case for it, they’re even less likely to get involved.
Other than some individual allies here and there, we’ll likely be largely alone in our opposition.
Same as we have been, mostly, in all the other culture war battles. (And Democrat-/Liberal-voting racial and ethnic minority Christians are useless, because notwithstanding their personal opinions, they never let such affect their continued solidarity voting for parties which most ardently fight on the other side in the culture war. So it’ll mostly be white Christians fighting the good fight, without even Моrmons on our side in this case.)