My 2 cents...
Originally Posted by SkinWalker
While it is true that polygamous societies tend to have more dysfunction within familial units (competition/marginalization between wives, neglect of children/wives, control of wealth, control of access to the husband and/or children, etc.), it cannot be overlooked that these societies also typically condone arranged and forced marriages even at very young ages and, even in monogamous marriages, they can be dysfunctional due to circumstances.
Still, the argument that if same-sex marriages are permitted, then multi-spouse marriages should also be permitted is fallacious. It hasn't been established that the former is necessary and sufficient to cause the latter. How does the ability to choose the sex of your spouse cause or require that others choose multiple spouses?
I think it's more a matter of "challenging the traditionally accepted definition of marriage." Once the definition is up for grabs, and the decidedly unconventional "same sex" model is equalized, it becomes much easier to argue for polygamy (which is, after all, at its core merely an expansion of traditional marriage favoring the male side). Polyandry would be in there too.
Of course if you keep the definition of marriage as "two persons" and simply omit the gender number restrictions, then you can say that. But again I think it's because you're rewriting the definition that then other models become possible.
Plural marriage also has the distinction of having been legal for long periods of time in many parts of the world in the past, vs. legally sanctioned homosexual marriage which doesn't have that historical precedent. So it would seem to come more readily to mind in debates about the definition of marriage than gay unions (even if homosexual activism is the entire reason we're discussing this at all).
And I think in any discussion of morality, we agree that something being legal doesn't automatically make it moral (or immoral) but people tend to naturally argue things should be legal or illegal based on some sense of what is moral or immoral, just or unjust, good or bad. So those arguing that gay marriage should be legal are presuming it is either morally good/right/just or else not immoral/wrong/unjust/bad (ditto with plural marriage) and therefore should not be criminalized.
In these sorts of discussions, my personal belief has lately tended more towards a libertarian "government hands-off" of personal relationships. Unless there is abuse involved, of course. Historically we can see where the benefit of heterosexual, monogamous marriage being protected and sponsored by law has come from, with regard to the protection of women (as a legally and economically inferior class for so long) and children. Now that we have DNA testing and some social safety nets, and because the social stigma of being born out of wedlock or raised in a "broken" (single parent or divorced and remarried) home has given way, one might argue that the time has come for less government involvement in such relationships, but I'm still thinking it through. But then the issue comes up as to whether the state ought to be responsible for public health and safety, and then it seems like there is some kind of "investment" being made in such relationships.