I think this thread is going to be the classical Social/Christian Conservatism vs. Libertarian Conservatism debate - so I'll give my two cents - it's time the two factions of conservatism found common ground, and the common ground is this; we all should oppose bad law! Back to Burkean Conservatism in other words!
In this case, I believe legalising polygamy would be bad law.
There is no such thing as "Libertarian Conservatism." There are Libertarian Rockafeller Republicans, and there are Conservative Goldwater Republicans, but Libertarians and Conservatives are two different animals witin the same party. When Reagan was a Libertarian Governor of California he signed an abortion bill into law and said he admired libertarians. When he embraced Goldwater's values, he dramatically strengthened the the Federal War On Drugs, and opposed abortion. A lot of liberaltarians like to hang out at this site, and generally they are ok, because they are typically strong on National Security issues, but let's be very clear about the fact that Libertarianism and Conservatism are mutually exclusive political philosophies.