Except that plural marriage sets up a MASSIVE load of potential problems that could tie up the legal system for years and years and years. Imagine plural divorces, multi-spousal insurance coverage, multi-spousal tax issues and on and on. The mind just boggles at how incredibly stupid allowing ploygamy would be here in the 21st century.
Let's face it. My husband could come home and tell me he wants a divorce. Or he might even come home and tell me he has a mistress. Or that he has a mistress and wants a divorce.
BUT - at least - I can go to bed at night knowing that he won't come home with a second wife! (Not that I'm anticipating any of the above to happen...)
Imagine if everyone was treated as an individual under the law. Then there would be no tax issues or insurance issues or divorce issues to burden the rest of society. The government should get the heck out of the marriage business. It's a private, and for many people religious matter, that the government has no business regulating, licensing, recognizing, etc.