The legislature is free to decide that a binary partnering union, presumably for life and for the purpose of creating nuclear families, shall have benefits while a non-binary union shall not. And, this is exactly how this will play out in every state. Believe me, I had to research the heck outta this to answer the legislative committees' questions in Vermont. They asked the same thing.
On the topic of polygamy, the US busted up the Mormon church and seized all its property until it promised to forego polygamy. The Supreme Court directly attacked the practice, calling it "pernicious" "repugnant" and a slew of other things.
The requirements for marriage can be freely set so long as it does not offend the constitution. Blood tests, prohibitations on consanguinity, and other requirements may be set, but the legislature could also require the couple live together for 9mos/yr or that they be a certain age, or a bunch of other things.
What the legislature cannot do is dole out legal benefits to one couple while not doling them out to another couple on the sole basis of the couple's gender makeup. What that does is fail to provide the "equal protections" of the law to all people regardless of gender.
[ 12-03-2003, 12:48 PM: Message edited by: Timber Loftis ]