"Darkness thus surrounds two worlds that are distinct but always associated. The same horror banishes the sexual function and excretion to the same darkness." Georges Bataille
As the old saying goes, "Variety is the spice of life". Our evolutionary history gives lie to the notion that humans are by nature, monogamous or alpha-male polygamous. Within the official ideological cauldron of patriarchal forms of marriage lies a roiling mass of social and psychological distress e.g. infidelity, divorce, one parent families and the blues.
The human race has had thousands of years of problems with monogamy, historical proof positive that we are not monogamous animals by nature but that we are forced onto this Procrustean bed by class dominated society's patriarchal legal, moral and cultural norms.
Marriage is a cultural tradition which flows into the legal structures of class ruled societies. Publicly recognised, lawful relationships between men and women have been like that since patriarchy was found to be a necessary link in controlling property and inheritance of same after agriculture and animal husbandry were developed circa 8,000 BCE. The mode of producing a surplus of wealth above and beyond personal need of the few is a foundation stone of class dominated civilsation and when conditions have been developed where it becomes possible for society as a whole to enjoy wealth beyond need, the material foundation stone of classless civilisation has been laid. It is when Capital as a social relation becomes recognised for what it is, a fetter on the freedom of the overwhelsming majority of individuals, that the rational bases of class society's foundations begin erode.
It's perfectly possible (not easy though) to change cultural traditions i.e. recognising gay marriage by educating, agitating and organising within the culture; however, the class who rules will not change over implementing these measures. In the modern era, that's the capitalists and their buddies in the landlord class. Cultural change is what is happening now. Supporting cultural traditions is what social conservatives tend to do. For instance, I support marriage between as many people who want to be married to each other (gay or straight). I even support monogamy as choice. Why not?
Legalise polyandry now! That would be my personal slogan. However, I realise that this is a cultural change most people would not support at this point in history. La luta continua. I do think that the whole cultural rumbling and grumbling about gay marriage contributes to the general historical transition social relations are undergoing nowadays, the transition from the monogamous (and polygamous) family, private property and the class dominated political State towards a free association of human beings who hold the social product of their labour in common and administer their collective wealth in grassroots, democratic ways, leaving their personal lives as a matter for individuals to design for themselves. Monogamy for most humans will ever lead to, "leading lives of quiet desperation" for the biological-evolutionary truth is that humans have an urge to satisfy their sexual needs within the variety of others they find around themselves. Thus, class dominated civilisation has become a fetter on freedom, an outmoded necessity worthy of sublation.