I think that the rationale behind allowing polygamous marriages is the exact same as the rationale behind allowing gay marriage.
I also think that the emotional justifications for people wanting to get married are the same, whether they partake in a gay, straight or polygamous relationship.
I'm not sure why Spider wants to act as if gay marriage and polygamous marriage are so vastly different - probably because he spent the last ten years assuring people gay marriage wouldn't lead to polygamous marriage, even though it was obvious from the start that it should and would.