The word 'marriage' is singular in connoting "a harmony in living," "a bilateral loyalty," and "a coming together for better or for worse, hopefully enduring, and intimate to the degree of being sacred." - Griswold v. Connecticut (1965)
Or to paraphrase in my mind the key words - "Harmony, Loyalty, Enduring, Intimate, Sacred"
And I realize this is over simplifying, but I think what would (good) Polyamory be if not these same goals. I know the slippery slope argument is often applied in regards to the leap from gay marriage to poly marriage, but in this case I think its potentially valid. I think that argument is used because it is within the scope of thought be possible someday, if not yet probable.
We aren't anywhere near ready, people will fight tooth and nail. I doubt I will see Heinleinesque Marriage formations in my lifetime. I do think it will happen, and what we work so hard at now will be at least be a little easier because it will be open and visible and recognized outside of our little tribes. At least I can hope so.
Just my thoughts on the day.