[firemanx]

As a counter point to your first item - the government now doesn't really tell you who you can and can't marry, at least from a religious perspective. However, in the US at least, marriage is often done both religiously and legally, which imparts a whole slew of rights, obligations, and benefits. From the legal and governmental perspective, polygamy is an abuse of this (see the documented cases of welfare abuse in Utah, for instance).

Back to the point though - you could get married to 10 spouses in front of God and your friends and never tell the government about it, and I doubt they would care (unless your cohabitation violated some health code or something).