I understand that for many of you this can be an off-putting and even upsetting topic, but that's precisely my point. If the mere thought of anything outside of monogamy is causing you unease, then you've already allowed the prevailing social mores to direct the way you think. Instead of shutting out the other side and making a biased predetermined judgement, allow me to present the best arguments for why polyamory makes sense.
Strictly speaking, "monogamy" refers to marriage to a single spouse, whereas "polyamory" refers to love of many people. Technically, there's no reason why someone can't be both monogamous (married to one person) and polyamorous (romantically in love with multiple people); in fact, it happens often during affairs. To avoid this confusion, know that when I say monogamy I mean monoamory (a less frequently used word) which means maintaining one romantic mate, and that marriage is not necessarily part of the equation.
Polyamory is Simply More Natural
Most proponents of polyamory begin their arguments with the idea that polyamory is somehow more "natural" than monogamy. They might argue that humans are genetically programmed to reproduce and diversify the gene pool, and that in our "natural state" humans would not be restricted to one long-term mate as evidenced by the behavior of our recent human ancestors and our ape relatives. Whatever the reasons may be, the argument is that humans are naturally predisposed to wanting polyamory. Note that when I say "natural" or "unnatural" I'm not using the terms in an ameliorative or pejorative sense. Instead, when I say "natural" I mean "congruent with biological desires and impulses". In this sense, homosexuality is just as natural as heterosexuality, for example, and polyamory then -- I will argue -- is more natural than monogamy. (Note that natural does not imply better either; my point is only that it makes no sense for something more natural to be viewed as something deviant or fey). As it turns out, the scientific community tends to agree with this position. Allen et al. make the following claim.
Although pair bonding appears to be a common human behavior, a survey of contemporary preurban societies ... provides no basis for the contention that exclusive long-term monogamy is or ever was the only bonding pattern for humans (1982).
The survey that the authors refer to is one found in Murdoch's Atlas of Human Culture. In the survey, 563 societies are cataloged, and of those only 17.8% showed any form of monogamy whatsoever (1981). What this suggests is that under the operational definition above, polyamory is more natural. This is because societies that are less fettered by layers of cultural mores, and are more in-tune with biological impetus, tend to align with the polyamoric paradigm, and thus polyamory appears to be more natural than monogamy.
However, if this line of evidence is not convincing enough to suggest that polyamory is (more) natural, then I will make my best efforts to appeal to intuition. One way in which we can intuitively know that polyamory is natural is by realizing that monogamy is hard. In fact, if we consider an instance of infidelity as a failure of monogamy, then monogamy is so hard that it fails between 30-40% of the time (Guerrero et al. 2010). It's important to keep in mind that that statistic refers only to reported incidences of infidelity and that actual rates are likely far higher. Moreover, that statistic doesn't take into account individuals' struggle with their urges. If we take into account the rate at which individuals want a romantic or sexual experience outside of their monogamous relationship but through willpower resisted the urge, then my contention is that the rate would skyrocket closer to 100%. Intuitively speaking, because monogamy requires individuals to consciously override their biological urges -- go against the flow, as it were -- then monogamy must be in some sense less natural than polyamory which requires no such unnatural restrictions.
Furthermore, in logical discourse, it is the claimant who has the burden of proof. Since monogamy is the policy that insists that biological urges be suppressed, and polyamory doesn't, the onus to demonstrate rationality is on monogamy not polyamory. If someone were to tell you that you can only eat one type of food for the rest of your life, you wouldn't need to make an argument for why you should eat other types of food, instead you would insist on an argument for why you can only eat one type of food in the first place. Likewise, polyamorists don't have the burden of proof here, monogamists do. Their policy is the one imposing a restriction, so they are the ones that must put forth a sufficiently reasoned argument for why I should listen. Though monogamists have supplied arguments, none are "sufficiently reasoned".
Arguments For Monogamy
Natural Isn't Necessarily Better
One argument that can be given for monogamy and against polygamy is that "natural isn't always better", and that nature is something that ought to be transcended. While it's true that natural isn't always better, it's not true that nature should always be transcended. There should be a reason for why something natural should be transcended. For example, peace is a transcendent state. War and violence are natural human behaviors that ought to be transcended for good reasons -- because people will suffer on widespread scales otherwise. Are there equivalently sufficient reasons for transcending sexual and romantic urges? If there are, I haven't heard them.
Monogamous Couples Are Stronger Than Polyamorous Ones
Another common argument for monogamy is that monogamous couples tend to form stronger relationships than their polyamorous counterparts. Comforting as this might sound, it's simply not based in reality. Monogamous couples are more fragile than polyamorous ones in a number of significant ways. The most salient of which is jealousy and insecurity. Monogamous couples are strained and often collapse at the first sign of jealousy or imagined infidelity, and almost always deteriorate after an actual instance of infidelity. On the other hand, polyamorous couples not only survive external affairs, but grow and prosper through them. Every new external romance reinforces the couple's notion of trust, respect, and openness, since there's no need for deception. Moreover, the sex life of the couple is improved as both members become better lovers through exposure to more diverse ways of having sex.
This mindset of strength and fidelity also causes loving relationships that are otherwise healthy to end abruptly and on hurtful terms. How many good relationships have ended due to a single moment of indiscretion when one partner succumbed to base needs? It's such a common occurrence that it's practically a trope of modern fiction. Under polyamory, a good couple will never be broken apart by something as petty as sexual infidelity; after all, how can a single moment of indiscretion override years (sometimes decades) of love and bonding?
Another way of phrasing the above argument for monogamy is to say that exploring romance outside of the main relationship weakens it by distancing the couple. Again, this is entirely false. Exploring romance outside of the main relationship reinforces it in two main ways. The first is by solidifying the belief that the partner you have chosen is right for you. In monogamy, there's no way that you can assure yourself that someone is not more suitable for you unless you go outside the bounds of monogamy. But in polyamory, you're constantly free to explore and convince yourself that you made the correct decision when you chose your mate. This feeling of confidence in your decision brings closeness and intimacy to the relationship. Additionally, by allowing partners to seek romance elsewhere, there's less pressure on the individual to constantly satisfy the other. The alleviation of this burden removes stress from the couple and, again, allows the individuals to grow closer together.
Other proponents of monogamy argue that monogamous couples are more genuine or more "real" because they are exclusive. In fact, in some ways monogamy forces its participants to be disingenuous. Partners must often lie to each other about their sexual needs and desires out of fear of inciting hurtful feelings such as jealousy or undesirability. Whereas this problem is much less prevalent in polyamorous relationships because partners feel much more comfortable expressing their desires for other individuals. In monogamous relationships, the suppression of these urges often leads to resentment, "midlife crises", and ultimately to marriage-ending affairs. On the contrary, it seems that polyamorous relationships are more "real" and genuine because their participants remain together out of choice and love even when granted access to numerous other partners, and not out of some sense of loyalty or promise.
Monogamy is Fundamentally a Selfish Policy
Additionally, monogamy perpetuates the myth that every urge and need that one has should be satisfiable by a single person. This notion causes stress and insecurity in people as they attempt to reach an impossible standard and fail. Let's face it, because there are seven billion humans on the planet there's a good chance that you're not even the best match for your partner, so why would you expect to be able to fill all of their needs? Why should you have to be able to fulfill all of their needs? By limiting your partner exclusively to yourself, you're doing them a disservice by not allowing them to have all of their desires met. This becomes especially pronounced in monogamous couples who have been together for a relatively long time. Sex becomes stale because humans enjoy novelty and get bored and jaded relatively fast. For obvious reasons, polyamorous relationships do not suffer from this defect.
A common objection to allowing external exploration is that "if I let my partner explore, he or she will find someone better than me." This is an instinct we all have. No one wants to lose something they love dearly. But if you truly examine that statement, you'll immediately see the selfishness in it. So what if your partner finds a person who makes them happier than you? If you truly love your partner, you would want them to do whatever makes them happiest. By adhering to monogamy, you're imposing a policy of mutual selfishness.
Not only is monogamy selfish towards your partner, but it's also selfish towards all of the people that could potentially be with your partner. If the person you're with is somebody you love, then chances are good that there is something about them that you value and treasure and deem worthy of love. In some sense you must consider your partner to be a great person. By not sharing your partner with others, you are depriving the others of whatever joy that your partner brings you -- you are hoarding love out of jealousy.
Put into formal language, here is a tongue-in-cheek argument for why polyamory is morally superior:
P1. If the most moral action is one that minimizes suffering and maximizes pleasure, and sex and love are among the greatest pleasures that a human can experience, then a relationship that restricts the amount of sex and love that a person can experience is less morally optimal than one that has no restriction on the amount of sex and love that a person can experience.
P2a. It is the case that the most moral action is the one that minimizes suffering and maximizes pleasure; and
P2b. It is the case that sex and love are among the greatest pleasures that a human can experience.
C1. Therefore, it is the case that a relationship that restricts the amount of sex and love that a person can experience is less morally optimal than one that has no restriction on the amount of sex and love that a person can experience.
Of course the argument isn't meant as a serious deductive proof of polyamory's superiority, but it is nevertheless fairly compelling and difficult to argue against (I'd gladly entertain some counterarguments). It's at least evident that monogamy is more selfish than polyamory, and is therefore at least a somewhat less compassionate way to live.
None of this is to say that polyamory is not without it's hardships. Overcoming the initial barrier of jealousy is difficult, and most couples are incapable of crossing that hurdle (in my personal experience, however, it was well worth the effort). Additionally, once crossed, there's no guarantee that jealousy and insecurity will not resurface later in the relationship. While I agree that this is an obstacle in polyamory, I'd like to point out that those issues also exist in great quantities in monogamous relationships as well. Those problems also only exist because polyamory exists within the context of a society that overwhelmingly instills the idea of monogamy and thus creates an environment of jealousy and insecurity.
In closing, polyamory may still carry a social stigma, it nevertheless offers a healthy and natural alternative to monogamy that is in many ways superior. Counter-intuitively, polyamory promotes security, trust, and intimacy in couples that employ it. Just as importantly, polyamory promotes traits such as selflessness and compassion, whereas monogamy encourages selfishness and jealousy.
At the end of the day, the decision between monogamy and polyamory is left up to the couple and the individuals that make it up. My only hope is to inform and educate others that there is at least one alternative and that it is viable.
Guerrero, L. K., Andersen, P. A., & Afifi, W. A. (2010). Close encounters: Communication in relationships. SAGE Publications, Incorporated.
Murdock, G. P. (1981). Atlas of world cultures. University of Pittsburgh Press.