Monogamy is defined by the OED as “the practice of being married to or having a sexual relationship with only one person at a time” (from Greek words meaning “single” and “marriage”). A certain type of human relationship (marriage, sexual involvement) is restricted by this practice to a single person at a given time: you must not enter into this relationship with several people simultaneously. The restriction is not typical of human relationships: other social relationships are permitted to have multiple simultaneous instances—friendship, parenthood, pedagogy, teammate, coworker, etc. People don’t enter into these relationships with the express stipulation that no one else may be permitted to be in the same relationship with the person in question—for example, you don’t expect a new friend to cut off all other friendships for your sake, keeping only unto you. These relationships are not deemed mono-relationships; indeed, it would seem preposterous to most people if someone were to take this attitude (“You are teaching someone else!”). A question must then arise as to whether the practice of monogamy is rational and desirable. What if a tribe inverted our practices and regarded friendship as mono and marriage as poly? That would seem arbitrary and unmotivated—are we so sure that our practices are not similarly benighted?
How monogamous are we really? We might distinguish strong monogamy and weak monogamy: the weak kind requires only that there be no sexual contact with another person, while the strong kind prohibits all affection and physical contact with others. The super-strong kind bans even affectionate thoughts about others, or being in the same room. We don’t normally insist on the strong kind, but it is a conceivable form of social exclusiveness. We make room for affection that extends beyond the marriage or romantic partner, though we grow uneasy when the affection crosses certain lines. Hugs and kisses are acceptable, but not kisses on the lips or prolonged hugs. We are not all that strict about passing infidelity of the heart, though that is close to dangerous territory for many people. The lines are blurry and the trespasses contestable, yet we continue to insist on the essential rightness of the practice. Is this rational? Is it good for us? Let’s chip away at it a bit. Suppose people distinguished different types or levels of infidelity: there exists a strict hierarchy of infractions, from hugging to handholding to kissing to genital contact. Each level has its own set of rules and laws with varying punishments that might be formalized in the marriage contract: say, fantasizing and hugging are acceptable but nothing more intimate. Not all offences are treated alike. Are these people monogamous? It is not a binary question: they are monogamous to a certain degree, in a certain respect, quasi-monogamous, monogamous-ish. “How monogamous are you?” “Pretty monogamous, but not, like, absolutely monogamous.” This is not so far removed from current human practices. But there might be difficult cases: suppose that a glance from a stranger at a distance of thirty paces could trigger orgasm if the stranger possesses a certain appeal. That might lead to a prohibition against looking at strangers in public places–and might be taken to be grounds for divorce, even though no bodily contact took place. A person who went around doing this habitually might be deemed hopeless marriage material. Special glasses might be recommended for the weak of will who can’t seem to keep their eyes down. On the other hand, the difficulty of policing such interpersonal stimulation might encourage a more tolerant attitude—after all, it never leads to marital desertion. A person might be partly monogamous and partly not—having a weakness for strolling around the mall on a Saturday afternoon but otherwise strictly mono.
There are more extreme possible scenarios. What if you are married to a person who has recently undergone brain bisection—aren’t you now married to two people? What if each self emerged on alternate days of the week so that you are actually having sex with distinct persons on successive days? Should you divorce one of them and decline to have sex on the days that person is in the ascendant? That seems an extreme response, though it is quite true that you now have two sexual partners if you carry on as before. Or suppose that brain fission occurs and the two halves of the brain end up in different bodies: you now have two individuals in the house where before you had only one. Should you choose one and reject the other? What if they are qualitatively identical? It would be unfair to deprive one of them just because of a pedantic insistence on monogamy. What if you underwent brain fission too, so that four people now roam around the same house? What marital practices should apply? It would surely be prim to limit marital relations so as to guarantee monogamy; a more fluid approach would seem more practical and not morally objectionable. But if that is so, there is the question of the number of selves inhabiting a typical human psyche: what if considerations of personal identity favored the view that we contain multitudes, or at least two or three distinct selves? Aren’t we then already in a non-monogamous relationship? If people began to accept that the self is not the unitary entity of traditional metaphysics, their attitudes towards monogamy might change correspondingly. You might prefer this kind of personal fluctuation as affording a degree of variety in your romantic life. No guilt need accrue; no jealousy need arise (as in “You like my other self more than you like me!”).
Let’s take it a stage further now that we are considering logical possibilities. Suppose an intelligent and sensitive life form to have the body of a snake or worm with segmentation; on each segment a distinct set of genitals are to be found, let’s say 10 in all. Imagine that the sets vary in their impressiveness (by some measure, say potency or pulchritude) with the better sets located nearer the head. Marriage exists for these creatures and rules about extra-marital relations are in force. The main rule is that one may only use the very best genitals for a designated partner, while the others may be used for extra-marital relations. It might be thought a serious violation to break this rule, meriting divorce or worse, while a tolerant attitude is taken towards lesser forms of genital wandering. This seems like a conceivable set-up and the attitudes described reasonable attitudes given the facts (maybe there is some variation in attitudes within this society). The notion of a binary practice of monogamy is alien to these aliens. There is a spectrum not an all-or-nothing choice. Things might be different for us if we displayed this kind of anatomical complexity: there would exist different categories of departure from strict monogamy.
What about jealousy—isn’t this an obvious problem for non-monogamous relationships? Yes it is, but so is it a problem for nearly all of human relationships. We all want to be number 1 not number 2 or number 23. We all want to be the one loved the most: the most-loved child, the most-loved friend, the most-loved pupil, the most-loved lover—or at least we are prone to such wishes. We tend to get jealous if we are not at the top of the list. We learn how to control this emotion to some degree, or we claim to, not sinking into despair when we find out that a cherished friend has better friends than us, or that we are not the most prized student in the class. We fear the loss or demotion of important relationships; we can be wracked with insecurity. This is, as they say, human nature: it is not unique to our romantic relationships. For some people, jealousy is easily triggered and hard to manage, in romance as elsewhere; for others it is less of a problem. But its possibility should not be used to insist on a strict and unrealistic ban on any forms of departure from monogamy. To do so is like saying that possible disappointment is a reason not to strive to succeed: it is true that disappointment often accompanies striving, but that is not a sufficient reason not to make the effort. Similarly, jealousy is natural in intimate relations, but it shouldn’t be taken as a decisive reason not to engage in non-monogamous relations or to prohibit them. It may even be a good thing in keeping us on our toes. As always, tact and good sense should regulate potentially jealousy-producing situations, but it is too much to demand that nothing jealousy-producing should ever be ventured. When you meet your dear friend’s best friend you must handle it with due humility and decorum, and your friend should take care not to rub in the disparity. So there is nothing special about the romantic case so far as jealousy is concerned. Maybe if mores changed the dangers of jealousy might be mitigated, especially if no loss of relationship were threatened. We frown on jealousy in non-romantic situations and try to get beyond it; we might come to feel the same way where romance is concerned.
Ask yourself if the idea of mono-hatred makes sense: the requirement that you can only hate one person at a time. Isn’t it possible, and not morally objectionable, to hate several people at the same time? Of course it is, so it would be silly to prohibit poly-hatred. It would be pointless and unrealistic. So why should romantic love be such that it is only permitted in the singular? It seems possible in the plural, like other kinds of love, so we must ask what rational grounds there might be for restricting such love to a single individual. Is it perhaps just a relic of outdated marriage laws, or a clumsy protection against disease, or a holdover from a preference for monotheism over polytheism, or a way to control women? Suppose a society had a particularly stringent code of monogamy—only one romantic love object per lifetime. Finding a new beloved upon the loss of an old one is deemed infidelity to the old. Serial monogamy isn’t enough; the monogamy must be temporally absolute. Surely that is a far too rigid and life-denying standard, despite its simplicity and purity. So why isn’t the prohibition on simultaneous romantic partners similarly draconian and life denying? Besides, it is not really empirically accurate, since monogamy as we have it is really a matter of degree and variation, with no sharp lines. It is a kind of abstract ideal not a practical compromise fit for the messy realities of life.
 What is intended here is not sameness of instant, which suggests threesomes, but overlapping of relationships. This allows for some flexibility, as when a relationship starts up again after going dormant and another person is meanwhile involved—you will not be convicted of infidelity in the interim. This makes the concept of monogamy more elastic than is often supposed, depending as it does on the question of when a romantic relationship exists.
 You might say that sexual intercourse provides a sharp line–either it occurs or it doesn’t. But (a) this is far too permissive for most people’s tastes and (b) we can manufacture possible cases that make it blurry (what if inter-genital stimulation is mediated by a suitable force field that operates over several centimeters?). The concept of “having sex” is actually not very well defined.