To me this is a silly argument, the people that support gay marriage will be fine with polygamy, and those whose first reaction is that it is different, will quickly come around to persuasion just as they came around to support gay marriage.
Gay marriage supporters are not suddenly going to get all shocked at polygamy.
“Gay marriage supporters are not suddenly going to get all shocked at polygamy.”
No, they won’t be shocked. BUT I suspect that they would hate it. If nothing else, it confirms that men belong with women.
They will be shocked when the polygamists get large enough to impose Sharia Law and behead them.