The topic of marriage privatization comes up once in a while -- the idea that government should get out of the business of regulating marriages, rather marriage would become a private contract. This way, gay marriage and poly marriage and such would totally legal without question.
I oppose the idea myself, though I'm not firm or clear in my objections. My biggest gripe is that it's associated mainly with libertarians, and I absolutely don't trust anything that's ever touched a libertarian.
Arguments that oppose marriage privatization:
Arguments that support marriage privatization: