In theory, most cultures agree that once you marry a person (or persons), you’re not supposed to sleep around outside of that bond.
In practice . . .
Adultery happens a lot, even in situations where the penalties for it are dire. We’re kind of a randy species overall; while individual drives vary, straying is a temptation for many. When it comes to the cultural level of how permissible this is, though, and how we as a society respond to it — well, the elephant in the room is the howling double standard between the sexes.
Women often face much more significant consequences for adultery than men do. When inheritance is patrilineal, it’s very important to know which guy sired a given kid; when power is patriarchal, women are (literally or figuratively) the property of their husbands, and infidelity is a slap in the face of men’s power. If divorce is permitted at all, then unfaithfulness is usually the #1 reason a man is allowed to get rid of his wife. In many places and times, nobody would so much as blink at him beating her; in others, she might be the victim of what we euphemistically call an “honor killing,” aka societally-sanctioned murder. (Though honor killings are not limited to cases of adultery. Rape victims are killed for the same reason, as are others who are seen as dishonoring their families. Including men, though much less often.)
Men are rarely held to the same standard. Sometimes women have the legal right to divorce their husbands for infidelity, but not always — and even if they do, they may face significant pressure to forgive his “mistake.” Elite men in particular have often gotten away with keeping mistresses as a matter of course, something their wives are expected to turn a blind eye to. Maybe the husbands are supposed to be discreet about it, not flaunting a mistress in public . . . or maybe they’re like the “Sun King,” Louis XIV of France, bestowing the semi-official title of maîtresse-en-titre on his main squeeze and installing her at court.
I should note here that although I’m speaking mostly of men having affairs with female lovers, homosexual affairs are not only possible, but in some cases were every bit as normal. In both Japan and Rome, there were stretches of time where the pimpin’ lifestyle was to be married and to have female and male lovers on the side. In ancient Greece, the general belief was that love didn’t really exist between husbands and wives; that was a political and economic partnership, and true affection could only happen between men. Unlike heterosexual affairs, these at least were unlikely to produce bastard children — look for them in next week’s essay. (Louis XIV’s brother, Philippe d’Orléans, paraded his male lovers every bit as publicly as the king did his mistresses, and this was accepted in part because it reduced the threat he posed to his brother’s throne.)
The idea of an “official mistress” shades over very rapidly into the ill-defined term “concubinage.” Depending on where you’re looking, that term can mean several things — up to and including what in English we would call civil unions, which is sort of what it meant in ancient Rome. At other time, where the boundary lies between concubinage and polygyny can be as much a matter of translation as anything else. But as we generally use the term, there are two interlinked patterns that tend to emerge.
The first is slavery. It’s very common for concubines to be the property of their master — which doesn’t necessarily mean they’re low-born, though it can. They may be captured in war, or sold off by their families as a political move, getting a relative into the royal or imperial palace. Even in the Roman sense of the term, a concubina was not the social equal of her partner, which was often why they did not have a full marriage. And I should note here that the both the female and male “lovers” of many Roman patricians were actually their household slaves. Whether they got called concubines or not, they were considered the sexual as well as economic property of their owners.
The second pattern is a differentiation of rights. When someone is referred to as a concubine instead of a wife, it usually means she has fewer marital rights than the woman with that latter title. In some ways, enslaved status is an aspect of this, but it also applies particularly to children and inheritance: often the children of a concubine are not eligible to inherit. Or at least, by default they aren’t; in ancient Mesopotamia, a barren wife could purchase a slave to be her husband’s concubine and produce offspring in her place. Historical Chinese dramas often mark the gradations of rank within the imperial women’s quarters by terming some of them concubines, others consorts, and one the empress — but one woman’s son might end up adopted by another woman of higher rank, especially if the higher-ranking woman is childless and/or the birth mother is dead.
So with concubinage in place, we kind of get a steady gradation from polygyny where all the wives have equal rights (though perhaps a hierarchy of seniority), to concubinage with a codification of lesser rights, to “official mistresses” given recognition at court, to mistresses whose existence is winked at, to truly illicit affairs. But again: this is virtually always about men’s infidelity. Because our societies are so often patriarchal, even in a matrilineal culture, women rarely if ever get the freedom of those first few tiers. There are quasi-exceptions; the medieval ideal of courtly love and the eighteenth-century Italian idea of a cicisbeo allowed for a woman to receive romantic attention from a man who was not her husband. But scholars debate whether courtly love was ever supposed to be physically consummated, and apparently the ideal cicisbeo was a gay man — not quite the same thing as a concubine or a maîtresse-en-titre. As for homosexual affairs between women, while they undoubtedly happened in the past as well as today, the historical documentation for them tends to be next to nonexistent.
But of course, all of that is history, and we write speculative fiction. In an imaginary society, you can set the standards wherever you please, whether that’s the court of la reine soleil with her collection of boytoys or a dystopia that mandates immediate death for all adulterers regardless of gender. The thing to bear in mind is that the standards tie in with major questions of reproduction, inheritance, gender equality or lack thereof, status and wealth, and more. Whether the situation is strict or permissive, the society around it needs to make sense with that.