[ZenGum]

Still, if we are thinking of civil unions, they come with various pension/insurance benefits. Polygamous unions could become tricky in that respect.
Suppose eight people get "married". Do they all get carer's leave when one is sick? How many mother-in-law's funerals can they attend? How would this affect welfare and pensions?
It would even be possible, by adding new spouses as old ones die, to keep a poly-marriage going indefinitely.
Not that these things should preclude poly-marriage, but I think they're interesting questions.