Third, it paves the way for civil polygamy, polyandry, marrying of beasts and inanimate objects, and otherwise destroys the institution of marriage, which does serve a public purpose since it provides for stable families for children.
It's a waste of your time to address that guy.
Considering the polygamy issue has been brought up by the Mormons, who are heterosexuals, that is a strawman arguement.
I don't even know what 'polyandry' is, or even if it is a word. How you can compare savage beasts too two consenting, intelligent adults is beyond me. And I don't even know what you got the idea of marriage to 'inanimate objects' from.