Heh, brother-sister marriages should easily be okay as long as the couple agrees not to have children. Perhaps one of the two would be required to get themselves fixed?
Polygamy is a problem where some/all of the participants have been brainwashed into doing it. When that happens, then one person (the brainwasher) has clear power over another (the brainwashed). It's obviously the case with a child bride, but one could argue that an adult woman could also be acting in the grips of brainwashing. So the issue becomes more complicated.
Finally, polyamory may be largely perceived in the same way. "No one would agree to that unless they had been psyched into it." It is hard to prove that any given person is not acting under some sort of mind control, but exposure to interviews with poly people may help other people to realize that a lot of (or most) poly situations truly are consensual.
Love means never having to say, "Put down that meat cleaver!"