Why is Polygamy such a taboo?

Those crazy West Coast hippies did it again. British Columbia had been refusing to prosecute known polygamists for awhile now. Shockingly the culprits are not new age commune dwellers, likely because they do not believe in the institute of marriage at all, but Muslims being sheltered by Imams in the community of Bountiful.
Arguments in favour of prosecution say that BC cannot refuse to prosecute crimes because they believe that the federal law is unconstitutional; they do not have the authority to do such a thing. Conversely, there is a lot to say about the law being unconstitutional. Though the law is often said to be 'protecting' women and children it was originally created to keep the Mormons from moving to Canada. Since the purpose of a law is nontransferable there could be some weight behind the argument.
Is polygamy really that bad? Like most relatively harmless personal social decisions, polygamy would probably be better if it was legalized. The closed Mormon and Muslim communities in North America are a direct result of such prohibitions. They shy away from mainstream society because their lifestyle is considered unacceptable. The cloistered communities then become a breeding ground for sexual misconduct. Small communities with an excess of husbands relative to wives drives down the age with which women are considered marriageable. Add this to the privacy that such couplings require to stay out of the public eye and organizations that check up on suspected abusers in two-person marriages are either unaware of what is occurring or not able to get into the community to stop it.
Though polygamy is not for everyone, would it not be better to have it monitored than to have it done in secrecy where the only candidate for your third wife is your second daughter?