There are enough complexities when just two spouses are involved.
I am a supporter of SSM, and a hetero. I would point out however that it appears you homosexuals seem to not "get" something that the antis have been trying to tell you on the matter as well.
You miss the point that it matters not what needs rewritten or how much needs rewritten, or how "difficult" it would be. The point is that if disallowing SSMs is argued to be unconstitutional on the grounds of discrimination, then it also applies to other forms of "marriages", and that includes poly marriages. If there is validity in arguing that the gender of the participants is irrelevant, than likewise, so is the number of "partners" in the marriage contract.
I find it fascinating that you (meaning most of the gays here, and NOT you in particular) argue valid constitutionally discriminating points when arguing your own situation, but stop short of allowing the same arguments to be made for marriages that you personally disagree with - which is you doing the exact same thing that you get all bent out of shape at others for doing to you. Your retort and comeback concerning poly marriage is a flippant "it's too hard because it requires a lot of writing", and you abandon the "constitutional ship" of valid argument.