How can you call it equality if it still discriminates against the polygamous? :)
Same-sex marriage is but a small part in the deceptively named "marriage equality". All people will be doing is playing whack-a-mole for decades (or likely longer) until all non-standard marriage configurations are finally enshrined into law. This is when they could simply privatize it and recognize marriage for what it is - a business/economic contract, and have marriage equality happen now.