Logically, I cannot see a reason why plural marriage should not be legal in America. However, I don't see it happening in my lifetime. There just don't seem to be many people coming forward to say they want this type of marriage. It strikes me as being very difficult to practice in the long term.
If, however, there were marches with lots of people, celebrities advocating for it, people on TV advocating it in persuasive terms, movies and TV shows about it showing the humanity of a plural marriage, then in time it would probably become legal. But you would need millions of people practicing it before you would make much progress.