The larger majority of human intimate relationships have involved two people. This couple lives together, likely have children together, have sex with each other, love each other, and don't turn to other people to fulfill any of those needs. However, it is possible to put together intimate relationships that involve more than two people. These situations are what this article is about.
PolygamyThe practice of having more than one spouse is called polygamy; the conditions of having more than one husband or wife actually have their own names (polyandry and polygyny respectively).
- Polyandry: One wife, two or more husbands.
- The advantages: In a sexist world where men work and women Stay in the Kitchen, this woman's children receive more care and nutrition; the presence of more than one breadwinner increases the likelihood that the little tots will live to adulthood. And if "raising my child to adulthood" doesn't sound like a difficult achievement, keep in mind that, before 1800, 43% of all human beings died during their first 5 years of life.
- Heir Club for Men rules may also play a factor. In mountainous regions where land is scarce, if brothers marry the same women, their joint household can inherit the family land together. Otherwise, the land would either (a) be split into pieces that are too small to support any of the brothers, or (b) be left solely to the eldest brother, with the others getting nothing.
- Alternatively, at varying times in varying societies, traditional men's work—such as farm labor—was vital for the family. Multiple husbands meant less work for each of the men and greater family productivity overall.
- Polygyny: One husband, two or more wives.
- The advantages: In a sexist world where men work and women Stay in the Kitchen, it allows women to get very high quality husbands. If you could contrive it that your child would be fathered by Brad Pitt, or Albert Einstein, or Johann Sebastian Bach, or Jeff Bezos, but to do it you had to become that man's second wife, would you? The benefits to your child would, after all, outweigh the inconvenience to you. Besides, these rich, successful men can probably provide for you more healthily than their currently-single competitors. This is the impetus behind animal herds involving one alpha male and a bunch of females: "he who is best and most fit (as chosen by natural selection) is the one I want my kids to be fathered by, for the sake of their futures."
- In a society where men are warriors or soldiers, they get killed at a higher rate then women, leaving a gender imbalance. Doubling up on husbands then becomes the only way for there to be enough to go around. Also, the widows of the dead warriors may become single mothers; this is a hard position for her and her children—especially since she's still expected to Stay in the Kitchen. If her society allows her to remarry, it's better for her kids. Finally, in a situation where large quantities of people were slain during the war, polygyny allows for rapid repopulation — it takes a woman 9 months to have a child, during which time a man could quite possibly have sired hundreds more children; and your antebellum society may well value this efficiency.
- At varying times in varying societies, traditional women's work was the most labor-intensive or provided the most economic advantage for the family. Some Native American / First Nations tribes went from monogamy to polygyny as American colonization became more prominent, because it became financially advantageous for the families to be able to produce more textiles and other goods that the women typically made, because these goods sold the best. In other words, multiple wives meant less work for each of the women and greater family productivity overall.
- One subset of polygyny is Concubinage, whereby the husband can have only one legal wife, but multiple concubines whose legal status is lower than that of the main wife, who often takes a position of leadership over the others. One prominent culture which practiced concubinage until it was outlawed in modern times was the Chinese. In North America, even though the mainline Mormon church banned polygyny in 1890, splinter sects practice it to this day. "Concubinage" might not the be word they would use, but it functionally works out the same way; a wealthy man might have one legal wife and one or more "spirit wives," where they're married in the eyes of their religious community but hold no legal status in their state of residence.
- Group marriage: And finally there's the group marriage, which is a marriage between three or more people of any combination of sexes and genders. These can get complicated, but in theory bring all the advantages listed above under one (very big) roof.
If you're looking at these ideas and thinking that they're primarily about practicality, well, you're right. The idea that a person might Marry for Love—a marriage where the partners' personal happiness is their main priority—is a relatively-new concept where human history is concerned. As recently as 1813, Elizabeth Bennet was still having to decide between marrying for love or marrying for comfort; and before the 1600s, marrying for love was basically not an option whatsoever. Instead, marriages had more to do with child-rearing, economic viability, and social/political alliances. This attitude has not entirely disappeared. Even today, "true love" only goes so far if practical considerations aren't met, like whether or not the partner can keep a job or if one wants kids but the other doesn't.
Incidentally, polygyny was legal in several ancient cultures, including the ancient Israelites of The Bible; Solomon was said to have 700 wives. It's still legally recognised in much of the world, especially in countries where Islam is the predominant faith. (Eritrea, India, Sri Lanka, and the Philippines recognize polygynous marriages among their sizeable Muslim communities, but not among other people.) Polyandry... not so much, and has mostly been practiced in the Himalayas.
PolyamoryThe next idea down the list (from most commitment to least) is the idea of polyamory, which is when you are in more than one committed relationship at a time, with the consent of everyone involved. This is a modern variant, focusing on love and dating.
Most people will tell you that, even if you can love more than one person at a time, it's hard to be committed to more than one person at a time, due to the selfish aspects of human nature. Polyamorists disagree. They don't reject commitment, but they do reject exclusivity, jealousy, possessiveness and the negative or limiting emotions that so often seem to come with it. The main difference between this and polygamy is that marriage is not considered a necessary part of a polyamorous relationship. Polyamory that focuses on long-term commitment is sometimes called "polyfidelity". (In practice, committed poly relationships rarely include more than four people at a time.)
Defined broadly, polyamory doesn't have to include romance or sex; it could mean that you want to form significant emotional bonds—of any manner—with more than one person (with the consent of all involved). In that sense you could argue that we are all polyamorists: even people who get married to only one spouse will still have emotional bonds with friends, siblings, parents, and children. Not the same kinds of bonds they have with their spouse, or so we hope, but bonds nonetheless. Generally, though, the world "polyamory" refers to romantic bonds, which are held has distinct from one's family and friends.
Also, if Alice is dating Bob while Bob is dating Carol, that doesn't necessarily mean that Alice is dating Carol. It could be that Alice and Carol are both romantically attached to Bob but not to each other, in which case Alice and Carol are considered "metamors" to each other. (This derives from the words "meta" and "amore", meaning lover).
Swingers & Open RelationshipsNow we start getting into a committed emotional relationship where the partners still having sex with other people—with, of course, the consent of everyone involved.
The root of this level is the understanding that sex and love are not faces of the same coin, and that one does not have to proceed directly from the other. This is patently Truth in Television, regardless of what the "Sex Equals Love" trope would have you believe. It's possible to have sex with someone you don't romantically love (see: casual sex, Friends with Benefits), and just as possible to love someone romantically without having sex with them (see: Courtly Love, Chastity Couple). Therefore, if you have sex with someone, it is neither implied nor required that you love them.
This takes two forms. "Swinging" is typically a pair activity; the couple, together, make choices to include one or more outside people in their sexual activity. In an "Open relationship," partners can make individual decisions about who they choose to be intimate with. The ground rules laid down varies from couple to couple: some only allow random encounters, while others say no randos and only sleep with people they know and trust. Others say "Whoever you shag, just use protection."
MonogamyAnd then finally down at the bottom is the idea of the two-person committed monogamous relationship, which is what Western culture (currently) considers the ideal. In this relationship it's against the rules to do anything sexual or romantic with someone besides your spouse/significant other. If you break the rules, you're cheating. If you change the rules, then you're not in a monogamous relationship anymore; you belong to one of the poly* categories discussed already.
Consent of partnersYou'll notice that none of these relationships give you the right to just go out and do whatever (or whomever) you want, without the consent of your partners. Generally, that's reserved for being single. All these relationships involve being committed to somebody—sometimes more than one somebody—and if you get together with someone when your partner(s) has told you not to, then you are cheating and that's that. This is where multi-partner relationships get tricky. Say you're in a relationship with Sam and Alex, and you then want to sleep with Charlie on the side. Alex is amenable, but Sam says no and won't budge (maybe Charlie was mean to them in high school). It's hard enough to get permission to do anything from one spouse; imagine having to clear your actions with two!
Open relationships can also fall victim to infidelity if one of the partners breaks any ground rules established, especially if outside play is only allowed under specific circumstances. Say, if one partner has a fetish that the other one isn't willing or able to indulge, that partner may have permission to sate their need outside the relationship, but they're only allowed to have sex with each other. Or a couple might have a list of trusted friends that they're allowed to fool around with; sleeping with one of them is okay, but hooking up with a rando would be cheating.
What should be pointed out is that none of these activities have to involve cheating. "Cheating" and "extramarital sex" (or "extrarelationship sex," though this is a less snappy term) are overlapping circles on a Venn diagram, but not identical circles. "Cheating" involves extramarital sex without permission. Extramarital sex with permission, on the other hand, is a significant feature of these relationships. Additionally, there is such thing called "emotional cheating," where you express loving emotions to a third party without the permission of your partner(s) — extramarital Courtly Love, basically. You don't have to even touch someone to break the rules; but, conversely, touching someone does not necessarily break the rules in and of itself.
Open vs closed relationshipThere is one last distinction to cover: an open relationship versus a closed one. A closed relationship is just that: whoever you're with, that person or those people are it for you for as long as the relationship lasts. An open one can have people added to it.
It's possible to have a closed poly relationship. For example, let's take the television series Big Love, where one man has three wives. If the marriage is closed, then that's it as far as sex and intimacy are concerned; Bill Paxton isn't allowed to get any more wives (or sleep with anyone else on the side), and his three wives are likewise limited to him. If he decides to sleep with a fourth character, that's cheating. Heck, if he decides to sleep with his third wife when it's someone else's turn on the schedule, that's considered cheating too!
With open relationships, partners generally agree that any outside encounters—whether it's with a friend or a rando—are not to go beyond sex. In this case, the primary concern is emotional cheating rather than physical. With swingers, they might only be allowed to play together (or within a swinger space) and not seek outside partners on their own without the other's knowledge or involvement.
Long story short, a "closed" relationship is one that is exclusivenote . The only question remaining is who you're being exclusive with.