A great deal of support for Gay 'marriage', politically, is merely dissecting the American Population and throwing a bone to each separate demographic, to win the election.
Many a Democrat, in office, must think marriage can only be defined as a legal union of man and woman. But to say so, would lose some votes, and the theory seems to be to garner all you can from every separate group that has a gripe.
As to polygamy, it did serve a Historical purpose, as it was used to show how very rich a man was, in that he could provide for a number of wives and kids, and helped the women and kids widowed and orphaned by the constant wars.
Right now, I cannot tell you how many women and kids my wife and I have supported, through welfare.