There were in the west three principles.
- There had to be two humans.
- Both of which should be alive.
- They have to be of different genders.
All over the world there are cultures that had something different to say about marriage. Allowing woman with more than one men, but more commonly the opposite. Sometimes marriages with animals or the death were conducted. These marriages were often highly symbolical and rooted in folk beliefs.
But since the west is growing to be more globalist. There’s no reason if one of these principles changes. Rule 3 which barred SSM, is a precedent for further change. Why couldn’t any other rule change? Well they can. If they get the popular vote it will change. With loads of islamic immigrants and the growing sentiment pro-polyamory and pro-polygamy during recent years this kind of change is likely. (swinging and open relationships keep getting more traction in modern society) First step will be a partner registration with several people. Likely this will be in one of the SJW’s nations like my own country the Netherlands. (maybe it’s already legal in some countries) Afterwards these types of registrations will be seen more in other non-islamic countries. Than there is bound to be a country that decides polygamous marriage doesn’t have to be illegal, because there are already people with these types of partner registration.
If this will happen is debatable. I think it might be hard for polyamory or polygamous people to get the majority to back them in their mission. But if some countries cave, others in the west are bound to tumble after them.