Polygamy’s nothing.
If we’ve arrived at a state in which the word “marriage” can be defined as anything we, personally, wish it to mean, then why can’t I “marrry” a sheep, or a dog, or a horse?
Didn’t someone somewhere marry a building?
When can I marry my dog? (Besides, she's always happy to see me and never gives me any lip.)
Traditionally, the consent of the other party is required. With animals, getting their verified consent may be an issue. However, if we are heading into a new Dark Ages, with heavy feudal overtones and a strong flavor of Might Makes Right, then I think the concept of consent will be waived. You can marry women who hate you — what does their opinion matter? And you can marry dogs and sheep — who will stop you?
‘Cause if marriage means anything, it means nothing.