i've got a nagging feeling that polygamy is more likely to be coercive and unconsenting than monogamy

That seems to not be an inaccurate representation of the reality in those remote parts of Utah where polygamy is practiced. Of course, I'm basing this on a few news reports that may, as usual, be overstating the case. Interestingly, I can't imagine that simply having multiple sexual partners and living together is illegal, but just the marriage part. And the Mormon church has officially made polygamy against their tenets (to the point where polygamists are excommunicated, IIRC), so it's really a small cult against the government, as they no longer really have anything to do with the official Church of LDS.

On the other hand, those potentially hyped news reports also claim that those polygamists are in some cases doing so incestuously, too, and that can't be good for anyone.