Polygamy is a whole different ball game to gay marriage, though. Polygamous societies are predominantly polygynous, so it degrades women and locks many men outside of relationships. Obviously piles of sexually dissatisfied men is bad news for society (don't laugh I'm serious), so I'm suspicious of promoting polygamy.
Given the way gay marriage has been promoted for the past 10+ years, I doubt people will consider those arguments valid. If I were to make an anti gay marriage post on Atlas or Facebook, one of the first replies would be something like "but they love each other, they should be able to get married".
A culture that bases its acceptance of gay marriage on "marriage=love" will have to accept polygamy too. Granted, there will still be inertia but it wouldn't be that surprising if our kids' or grandkids' generation feels the same way about polygamy that millennials do about gay marriage.
