By nature, people are small group animals, kind of like apes. By nature, alpha males dominate the sexual system, with unlimited sexual access to females and subordinate males fight their way up the pecking order.
But people developed culture, long after our instinctual nature was set. Marriage is part of human culture and marriage system try to control, channel and direct our instincts to avoid conflict and increase chances for children's survival.
In the world of culture, ideas about marriage grow and evolve. Older systems make the old instincts of male power and dominance official; but newer systems, for a whole host of cultural reasons, move toward gender equality, mutuality and reciprocity in marriage. Old systems are systems of male ownership of women; new systems are mutual obligations.
I think that monogamy arose first as a way to limit the effects of competition among males for women (instead of the alpha male having access to all the women for the brief moment that he was king of the hill, pair-bonding was developing -- almost everybody had a sexual partner -- more sex, less fighting, with the added, but unplanned bonus, of greater genetic diversity.) But in more modern times, monogamy serves the interest of women by providing a more stable source of support for periods of pregnancy and infancy. Now, the push for monogamy is linked clearly with gender equality in those parts of the world where polygamy is culturally sanctioned.
Among heterosexuals in the West, the cultural debate is between "Promise Keepers" who want continued male dominance of the marriage and the "Equal Partners" who look toward gender equality as the underlying paradigm of the marriage.
As that strand of the Western religious tradition that is most committed to the radical equality of all souls before God, liberal religion knows where it stands in that cultural debate. We have a theological commitment, I think, to marriage as a set of mutual obligations between equal partners.
So, if you have been following these arguments about the theology of marriage I have been making, you see that I am proposing, so far:
1. marriage is not a natural act for humans, but a cultural construction which channels and directs our instincts toward socially beneficially ends.
2. The first principle we uphold is commitment, life long commitment between marriage partners. It is this principle that led to liberal religion's support of same-sex marriage.
3. The second principle is equality between marriage partners and turning away from our instincts toward male dominance.
My next post in this series will further develop my point about equality, specifically the fact that in the post-industrial West, the paradigm of domination and subordination has floated free of gender, to appear in many forms in all sorts of relationships.