Any more than anti-polygamy laws violate due process?
Correct. If ten people (regardless of gender) want to get 'unioned,' let them create a word to describe their relationship and try to get an amendment passed that would recognized their class and afford them protection under the Civil Rights Act.
(Jeez, I was hoping everyone would understand what I was implying -- that such an amendment would not pass, but the process would be there for those who chose to pursue their folly and would thereby be unable to claim deprivation of rights.)