Regarding your discussion of polygamy, there are some interesting economic issues involved here. A good place to start is Gary Anderson and Robert Tollison (1998). "Celestial marriage and earthly rents: Interests and the prohibition of polygamy." Journal of Economic Behavior and Organization, 37(2): 169-181.
The basic argument is that, as long as existing wives have direct or indirect veto power over new wives, polygamy is generally welfare increasing for women, but welfare decreasing for men in the aggregate (basically, polygamy leaves a shortage of female marriage partners). Anderson and Tollison argue that men used their greater voting power to induce the government to crack down on polygamy among the Mormons.
Interestingly, if there's reason to believe that members of one of the sexes are more likely to enter into legalized homosexual marriages (empirical question, I guess), then there would be a similar disruption in the partner markets, if sexual orientation has any plasticity on the margin (another empirical question, I suppose; Posner discusses some of the evidence for "opportunistic homosexuality" in his Sex and Reason book). It seems to me that there are significant political economy forces (similar to that described by Anderson and Tollison) which push against any such disruptions.
I know the marraige partner market is highly competitive, and this just serves to illustrate the power of regulation. I wonder, though, given that a significant number of these transactions take place in the black economy, if such regulation is counterproductive.