This is what I don't understand (and I'm being serious): why do these people booing him oppose polygamy? We're not talking about bestiality or pedophilia here.
Honestly, what makes someone support gay marriage but not the marriage of multiple parties? If your standard is that something should be a "right" if there is a group out there that wants it, and it's based on consent (and not coercion), then it seems hypocritical to deny people that "right".
Especially considering gay marriage is basically without precedent and polygamy has been established in various cultures around the world for thousands of years.
I have no problem with polygamy per se, so long as all of the participants involved are consenting adults- which is generally not the case. Polygamous sects routinely arrange marriages of their teenage daughters to elders in the group, while expelling many of their teenage sons from the community because of the simple math problem. If the girls are over 18, and entering the arrangement of their own accord with no coersion, then sure, why not?
And should those marriages be legal? This would not be to the advantage of the polygamists, as the wives would no longer be able to claim welfare benefits as "single mothers".