In fact, you could argue that there is an even better argument for polygamy than for same sex marriage. For one thing, there’s a long tradition (just look at the heroes of the Old Testament.) It’s also intimately tied to religious practice, which means that by prohibiting polygamy, we might also be undermining the “free exercise thereof.”
Why should we impose our values on others?
Now, you might say that there is historical evidence to support the fact that polygamy is bad for women and children. This is sophistry. The truth is that right now about half of all marriages end in divorce, and lots of kids are already struggling, so it’s not like traditional marriage is a panacea. Besides, nobody is forcing you to be a polygamist. This is a choice.
There are practical reasons, too. It’s harder and harder these days to make ends meet. As a man, I can only imagine how much more efficient it would be to have one wife in the workforce and another wife at home with the kids.