People are not naturally monogamous. In the state of nature, before culture, we lived as herd or pack animals with a sexual system similar to apes, chimps, horses, dogs etc. Alpha males, alpha females, subordinate males and females, unequal sexual access.
With the rise of culture and religion, people developed more ordered sexual systems, including marriages of all types. Religion is also a product of the same development and, among other purposes, provides a moral and ethical authority for the new sexual systems.
The fact that we are, by nature, one way, and have chosen, by culture, to live another way is the source of our divided selves – temptation, sin, etc.
Marriage systems keep changing to fit the needs of the culture, while retaining a moral aspect – however the marriage and sexual system changes, humanity cannot go back to the state of nature regarding sexuality, hence there will remain a moral component to thinking about sexual relationships and activities.
We live in a time of tremendous cultural upheaval – the changes in global economy, post industrialism, global urbanization etc. etc. means that much is up for grabs. This is creating a moment of creative crisis for sexual relationships and marriages as we step into uncharted territory. There is more freedom about life arrangements than ever before, and hence more potential healthy and unhealthy consequences.
The downside of the present moment is seen in a wide variety of social ills. The upside is seen in a wide variety of good things happening. It would take lifetime to list them and sort them out.
Our religious perspective, the Living Tradition of liberal Religion, has one overarching direction: the replacement of a religious and cultural systems that are based on external authority and cultural conformity with religious and cultural systems based on internal sources of authority and conscious covenant making between people. To counter the natural chaos, should people rely on coercion, or on covenants? We are heirs to a tradition that consistently reinterprets the Western religious heritage as culminating in personal freedom and covenantal community formation.
I also see signs of a return to a technologically facilitated state of nature, in which no commitments must be kept. On an individual basis, the moral self-discipline to keep commitments of sexual fidelity is constantly challenged. It feels natural and liberating to act upon all of one’s sexual desires and fantasies.
As tempting as it might be, a return to a state of nature of uncommitted sexuality would be a nightmare. The highest human potential, what we seem intended to be, is the result of the process of enculturation. Love, solidarity, empathy, compassion all grow in the cultural space created by our agreements to not just live together, but to live together in arrangements that we learn, remember, teach, pass on and make into tradition.
Because we are liberals, we take a critical view of traditions and are willing to experiment and revise them in an evolutionary process of cultural development.
We are not just “on the side of love.” We are on the side of covenanted and committed love, because we know that uncommitted love does not reach its full depth and meaning.
Our theology of marriage is that we are in favor of it, because it is the current cultural form of covenanted love. We are in favor of it for not only heterosexuals, but also gays and lesbians. My critique of our weddings is that we focus, often too much on love, and not enough on commitment. People fall "in love" all the time; we honor when they move from attraction and affection to lifelong commitment, because we know that attraction and affection are not enough to last a lifetime. More perfect love comes in the aftermath of disappointment and the temptation to quit.
When it comes to extending marriage rights to more than groups of two people, we say “no” at this time. While much of the rhetoric about polyamory is full of the language of commitment and fidelity, we are not sure at all that the small polyamory movement represents a step forward to wider forms of covenanted love, or whether it represents a step back toward the state of nature from which humanity came. The crux of the problem to me is that Polyamorists seem to claim that some people are, by nature, “poly people”, and hence, unable to make successful monogamous commitments. But, my understanding is that all of us are naturally “poly people” in that by nature, we are not monogamous. If that is true, than polyamory becomes a rationale for eroding the cultural commitment to monogamy, not just for a few, but eventually for everybody. Time will tell on where multi-partnered relationships will lead. The social and culture environment in the USA these days permits almost any sort of social experimentation.
Final theological note: I am a Christian with a modern and scientific outlook. When we look at that moment in prehistory when humanity began to create arrangements that were not instinctual, but learned and taught, one might say that it was an intervention by God, that created humanity.
One could also say that humanity projected onto the empty sky a deity who directed their activities to add weight to the traditions being created. Either way, the religious traditions have provided the moral authority behind the human culture that we create. As such, they have played reactionary and revolutionary roles at different points of history. Those religious traditions speak to the most vexing areas where we have to guard against following our instincts back to the state of nature: sexuality, money, wealth, power and violence. Christianity is a vast system which explains this contradiction in our natures, and more importantly, offers deep insight about how to live with the inevitability of failures and shortcomings, of sin.