When I've heard the claim that gay marriage wouldn't lead to legalization of polygamy, all I could think of was that polygamy has a far greater claim. I believe it was the norm in much of the ancient world, and is still observed in at least some Muslim countries. No society has ever had gay marriage.
I saw a little thing in the paper (I thought it was today's paper, but I can't find it) about some people pushing for "group marriage," i.e., not polygamy or polyandry, but sort of open-ended, for people who are married but want to branch out without being considered unfaithful. A woman quoted in the piece (just a short one; not much more than a paragraph) said she herself has two "OSOPs" (other significant other persons) besides her husband.