I’m not sure it is far-fetched to think that polygamy could be legalized in the U.S. After all, same-sex marriage is on the march. So, the very definition of marriage is being subjected to examination. The secular notions of marriage could surely embrace polygamy, so you could conceivably have a nexus between Mormons and athiests on this issue, but could not have any common ground between those two groups and Christians.
I agree, though, that a re-run of Mormon polygamy would destroy the LDS as a functional unit. Most LDS women simply would not stand for it. It would wreak havoc within the Mormon church, and leave standing only small groups of fundamentalist Mormons, such as we have now. We must keep in mind that women, especially American women, are not the obedient flowers they were 150 years ago. Then, they had no options apart from marriage, husband, children. Today, they have as many options as men do, so that the idea that women would put up with polygamous husbands the way they were forced to within Mormonism in the mid-19th century, is a crazy idea.