As a student of anthropology, I think a lot about cultural constructs which permeate our lives, most of which are pretty invisible to us most of the time. From an early age I was fascinated with the various institutions of religion, as well as the many more personal spiritual ideologies people espouse, and the various psychological and sociological structures which surround them. Later on, I started thinking about similar aspects of how we think about sex and relationships, and eventually found many similarities between how we think about gods, spirits, and sex. This is no surprise since one of the best ways for religions to hold our attention is to demand certain behaviors is to hold us hostage with fears about our deepest inclinations; sex is a great example of this.
In religion, there is this idea of atonement. It comes in many forms from many theological systems, but it is basically the way that we come to make amends with some supernatural or natural power. Whether we have to deal with a fundamental brokenness in our nature, some separation, or lack of enlightenment (to only scratch the surface of ways religious ideas deal with this atonement thing) from the power we seek atonement with, there is a set of actions and beliefs which we must do or have in order to reach some ideal relationship with the universe, deity, or ourselves.
It is my view that the religions which survive best find a balance of difficulty and comprehensibility in the ideals it sets up. Adherents must be, like the Tantalus of Greek mythology, perpetually reaching for this ideal of atonement which they cannot really achieve, but it must be something they can imagine as a logically possible thing to have. Sure, people can think that they have achieved the goal (as many Christians believe they are saved), but the scriptures and religious leaders will always mention that this is pride, or something similar in order to keep them in check.
Opposed, conceptually, to atonement is some detriment presented as part of our nature or circumstance. We stand unenlightened, sinful, or separated from some god(s), knowledge, or understanding and we will remain there until we atone, repent, or whatever must be done to solve this problem, heal this sickness, etc. As Christopher Hitchens said many times,
Even the most humane and compassionate of the monotheisms and polytheisms are complicit in this quiet and irrational authoritarianism: they proclaim us, in Fulke Greville‘s unforgettable line, “Created sick — Commanded to be well.”
The bottom line here is that there is a tendency in human worldviews, whether religion or otherwise, to present a highly unlikely ideal against some much more likely, and often repugnant, set of behaviors or beliefs which we must be encouraged away from.
The sin of non-monogamy and the atonement with The One.
There is a mainstream view of sex and relationships, here in the modern west and most other developed nations, with monogamy as the ideal relationship type. The majority of cultures have some version of this practice, and it’s major competitor is some kind of patriarchal polygamy. Polyandry or true sexual/relationship equality is rare and considered aberrational when it occurs. It took quite a while before we would have a sexual revolution, and with it true freedom started to become part of our cultural consciousness.
And yet even still there exists within our sexually liberated world a distinction between studs and sluts; men are expected to be promiscuous, women are often valued for their “purity.” These promiscuous men and these sluts are expected, or at least encouraged, to eventually outgrow this part of their life and find The One, or at least settle for A One.
For most people who have a period of sexual liberation, it ends with the attempt to reach an ideal of monogamy. Men and women may be expected to have sexual experiences in their teenage years and into their 20’s, but eventually most people expect them to settle down. “Settling down” means taking relationships seriously, and usually means exclusivity, marriage, and monogamy. So while we are liberated as a culture in terms of having sex before we “get serious,” get serious we should, because seriousness means exclusivity and exclusivity is good.
The fallen circumstance, or nature, which even our progressive culture patiently tolerates for is one of promiscuity. But this sexually liberated part of our lives is held against a stable future ideal of monogamy. The holy grail of relationships, The One, is presented against the superficial and regrettable reality of youthful promiscuity. This One is The person with whom we can have a real relationship, rather than failing perpetually hopping from one insignificant relationship to another (sometimes at the same time!).
A mnemonic device I learned years ago about the word atone was that you can break it down into at+one. In other words, especially for many Christian traditions, the goal was to work to become at one with some god or another. All of our other inclinations, not having to do with this atonement, should be secondary to that relationship of working towards chasing that ideal, because nothing was more important than that. Monogamy has taken a similar place in our culture as that ideal of religious atonement; the sinful and superficial world of sex, lust, and other failings of human behavior are presented against an ideal of monogamy. That is, even liberal society maintains this ideal, even though that liberality allows sexual promiscuity, co-habitation before marriage, etc. Anything that looks like monogamy, even if it isn’t really marriage, is what we should be striving for. The difference here between mainstream conservatives and liberals on this issue is how we get to monogamy, not whether that is the goal.
This shows me that culture tends to be truly human (all too human!) and tends to have worldviews which are conservative even when we are progressive (I actually argue that today’s liberals are tomorrow’s conservatives, because the mainstream is largely conservatives concerning ideals). We conserve ideals, even as our values shift. So, even as we become increasingly liberal as a society in terms of seeing redemption and atonement in looser and looser terms, we hold onto the ideal itself. Liberal views about the supernatural and what we should be doing with our lives changes in terms of the details of the path to get there, but the destination does not really change. This is one of the greatest failings of most of mainstream liberal culture; it does not seek to question the ideals, assumptions, and goals of our worldviews.
Ideals Worth Wanting
What should be the strength of progressive culture, or perhaps a radical culture, is the re-valuing of our values. We need to evaluate what is worth valuing, not what we should change in terms of how to get to our Heavens, Nirvanas, or other paradises whether they be otherworldly or physical. The question is not how we can get to paradise or what we are allowed to do before we settle into monogamy, the issue is why do we value such ideals? Why is being at one with some supernatural power good? Why is monogamy the ideal?
I don’t think there are good answers to those questions, except to say that perhaps those things should not be ideals at all. With religion, atonement is merely an impossible goal, set before us to tantalize us and keep us striving and behaving within acceptable boundaries. Monogamy is no different, in that the only way to achieve it is to pretend as if our ability of love is so limited, and our sexual desires so parochial, that we force ourselves into ideal relationship expectations while repeatedly failing in thought if not act. There is no reason to set us up with impossible ideals which make no sense to value, whether with gods or monogamy, when we have real ideals to inspire us.
A skeptical approach to reality brings us to an informed and skeptical atheism, and allows us to love the people we love, the way we want to love them, in order to live authentic and rewarding lives. And while we may never be ideal skeptics or lovers, we can at least have ideals worth wanting.
Skepticism, Properly Applied 
A number of things I would like to address:
Not all religions are “separate paths up the same mountain.” We’re talking a bunch of mutually-exclusive mountains with many paths up those. There’s this dismissive, incorrect, retrospective way of applying a Christian Monotheist film over all our analyses of other religions, particularly pre-Christian Polytheist religions, as the Christian worldview is the one with which we are most familiar, and many scholars of the 19th and 20th centuries more often than not colored their interpretations and findings with their Christian biases and beliefs, trying to find antecedents to Monotheism (see also Champollion and E.A. Wallis Budge, and their far-reaching butchery of Egyptological Studies). Whether one takes stock in any theological package, to lump all religions together in such a universalized, one-size-fits-all fashion is to be ignorant of cultural contexts and the ins-and-outs of the theologies one is analyzing or criticizing.
Not all religions, especially pre-Christian Polytheist ones, have concepts of “Original Sin,” or an idea that “suffering and self-denial = enlightenment and access to Paradise.” Not all demonize the maintenance of multiple spouses/sexual partners, or consider sex to be an inherently sinful act. A physically contaminating act (even in Antiquity, people generally understood the dangers of exchanging bodily fluids, and how gross and germ-ridden that can sometimes be, and preferred to abstain from exchanging contaminant-ridden bodily fluids on certain holy days), but not a “sinful” one.
Take Ancient Egyptian religion, for example. No concept of Original Sin. The Gods Themselves at times deceive, lie, cause conflict, are imperfect. Ra (generally Ra, as per generally-accepted State religion post-Unification, but various Nomes and cult centers had multiple Creation myths for various chief deities), Who willed Himself into existence for whatever reason from the primaeval waters of Nun, created an imperfect Creation out of a very humanlike loneliness, and it soon went beyond His control. He created other Gods, in many accounts by the acts of sneezing and masturbation, to aid Him in His attempts to bring order to His Creation, but His control only continued to lessen as a result. These new Gods were creating other Gods beyond the extent of Ra’s control (see also Geb and Nut and Their offspring, “the Children of Disorder,” Wesir, Aset, Set, Nebt-Het, and Heru-Wer — not to be confused with Heru-sa-Aset, Aset’s son, Who comes much later). These “Children of Disorder” inspired mankind to rebel against their Creator — it was not the fault of mankind, for being “evil” by nature. All of Creation was/is imperfect, much to the regret of the Gods. Ra makes a point of saying, “I did not [intend to] create men with evil in their hearts.” See also Dimitri Meeks “Daily Life of the Egyptian Gods” for a more detailed analysis of Egyptian Theology.
None of the Gods in Egyptian religion are all-loving, all-good, infallible, and so on — qualities ascribed in Christian Theology to the Christian God. They kill, They get utterly drunk, They can get jealous and impulsive, They can lie and steal, They can be swayed by love and compassion. Only when we get closer to the end of Polytheism in Egypt, during and after the Roman Occupation, when the cult of Amun was most prominent, do we begin to see qualities of a Universal God Who is not only more emotionally detached, but omnipresent, omniscient, all-good, etc.
Likewise, access to the Afterlife/Paradise in later periods of Egyptian History, when more social classes were considered able to have a chance at gaining access to their own piece of Eternity, was not gained by self-imposed suffering or atonement or “virginity” of soul, which are the Christian requisites of eligible for eternity in Heaven. Various funerary texts, such as Coffin Texts and Books of the Dead, were choc-full of spells and incantations designed to alter an irrational reality present within the Uncreated Universe, and deceive or defeat various Otherworldly entities, Gods and demons alike. One finds “negative confessions” in Books of the Dead in particular, which were not declarations of “I am a pure person, I have never done these things, so you should let me into the Afterlife.” They were lies with magical powers; they were intended to deceive the guardians of various gates, defeat monstrous demons, and fool Gods Whom the deceased had to face in the Duat. In short, one had to be very, very good at illusion, deception, and lying — better liars than the Gods Themselves had/have the capability to be. So much for “atonement.”
From the examination of funerary texts alone, we already see a profound incompatibility with the Christian worldview and religion. Suffering was seen as part and parcel of the human condition within Egyptian worldview, being part and parcel of an imperfect Creation, but suffering and its self-imposed amplification was not traditionally emphasized as a vehicle toward Paradise or enlightenment. The Ancient Egyptians loved life and its pleasures, and very much wanted it all to continue.
In Egyptian culture, polyamory was not unheard of. The Gods Themselves often had polyamorous relationships and multiple spouses, if one wishes to look for a Divine precedent that showcases cultural mores, though marriage itself was a civil rather than religious affair in Ancient Egypt. For a man to take several wives, if he were wealthy enough, was socially acceptable and not considered “sinful” — it just didn’t happen often, as maintaining several wives was financially impractical. To go outside one’s marriage(s) was generally frowned upon, as is the case with any breaching of formal vow, confidence, or legal contract (even in Modern polyamorous relationships, there is a general understanding and agreement between the parties involved, and to breach confidence with partners by seeing others beyond their knowledge and approval is just as frowned upon). But having several partners was not frowned upon then, and in my own personal view, so long as all partners are happy and consenting, shouldn’t be frowned upon now.
Additionally, women, as a general rule, had extensive rights, including the ability to divorce of their own accord and own property, and were not considered “evil vessels that spiritually corrupt men,” as is the case within Christian traditions.
My apologies for the preceding wall of text. I *think* I understand what you’re trying to say, within the limited scope of Modern Western (European and American) cultures, and I agree that monogamy isn’t right or conducive to happiness for everyone. Whatever the goal of existence may be, it ought not to be “follow these contrived rules of courtship and settle down with this one person.” However, it’s neither accurate nor equitable to paint ALL religions with a broad brush. A lot of people in the West do maintain non-Monotheist belief structures, a good deal of them Polytheist, and most of those people don’t maintain a sense of “suffering = enlightenment” or “I am a bad, unclean, slutty person; I must atone.” It’s simply not part of those worldviews; such ideas are entirely alien to them.
Furthermore, I hardly think religion is to “blame” for monogamy, even Monotheism. I mean, King Solomon had, what, 700 wives and 300 concubines? And there were plenty of other instances of polygamous arrangements in the Old Testament. Not to mention the regional tribal Arabic and Turkic mores that carried over into later Islamic regional cultural practices during the Early and Central Middle Ages. And even the most devout of Western Latin Christian Frankish Kings maintained a plethora of wives and concubines at any given time. There were and are just as many pre- and non-Christian cultures around the world that generally favored monogamy for whatever reason. I’m not sure what the ultimate driving force behind monogamy might be. While Monotheist religious beliefs *can be* used as a *justification* for it (poorly, given the content of the Old Testament), I sincerely do not think it is one of many contributing causes, much less a prime mover.
I hope you understand that I was analyzing the conceptual similarity between relationships and religion from a western point of view. I have studied religion pretty extensively, and know that my analysis would not be true for many other ways people make religion. However, this was a post about western concepts.
My posts are too long as it is to concern myself with being comprehensive to the extent I would have to be to differentiate other religious views from my ideas. II will say that I do believe that much of religion, especially in the west, does tend to have similar structures of thought which loosely fit into my description above, but certainly not all. I think the analysis of ideals versus falling short of said ideals is pretty common in world religion, although it is expressed in many different ways.
My central point was that in western culture, there is a distinction that is expressed in Christians terms as atonement/redemption versus sin. In other religious traditions, different terms are used and the ideas differ somewhat or greatly, but a basic conceptual distinction of “what we should be doing” versus “what we should not be doing” is pretty universal. My analysis was to point out that this conceptual distinction is logically structured much like how we distinguish between monogamy and non-monogamy, not to paint all religions with a broad brush and say that monogamy is like that too.
The broad brush approach to religion here was done out of interest in post-length and to not become overly complicated, not out of ignorance.