You have formed two separate questions, even though you are generally asking only one.<quoted text>
I find it difficult to see the logic of defending monogamous marriage as the historic norm when the laws of many states have already departed from the principle that it is heterosexual, monogamous marriage that is essential to social stability.
Put bluntly, if heterosexuality is no longer legally, morally or socially relevant to marriage, why should monogamy continue to be so important?
1.) Should monogamy socially be relevant to marriage? And the answer is "I don't know". Other cultures, currently and historically, have embraced non-monogamous marriages. Obviously it worked (works) for them.
2.) Is monogamy a VALUE for First World, industrialized countries? And the answer to that question is an almost unanimous "YES!"
For whatever reason, we in large continue to value monogamy--even gay couples value monogamy.
Maybe it's a statement about our value of a "love-based" marriage system. Whereas in polygamous marriages, there is less of a "love-based" quality, and more of a utilitarian basis.
Monogamy is not necessary for social stability--as you suggest in your comment above. If that were the case, then every culture or tribe or society that has a polygamy based marital system should have failed. And they haven't.
But monogamy is what our culture values. It places great importance on one-to-one love and trust. When that love and trust is breached through adultery, then the marriage can fall apart.
But that is a luxury that we have. Women are no longer solely dependent on men. Our laws protect women (and men) from the heartache of adultery (which is, I believe, a form of attempted polygamy).
As long as our cultures continue to value monogamy--love and trust between two people--then it is highly unlikely that polygamy will gain a foothold.
Who knows... Maybe polygamy will become legal here some day.
But it won't be because gays were given the right to marry, anymore than it won't be based on the legalization of interracial marriage.