I have an earlier answer that might interest you!

• Medieval literature treats adultery as common and not especially a bad thing. Does this actually reflect the incidence and social treatment of adultery in medieval England?

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The wild sexcapades and bridled courtly passion of medieval popular literature seem to come from a different world than the early medieval Church penitential handbooks. Or perhaps you prefer 13C theologian Thomas Aquinas writing that adultery represents a special deformity of lust. But between fantasy and prescription (or one might say: between writers' fantasy and the Church's fantasy), there's a lot of room for negotiation and change over time.

Medieval western Europe inherits Roman and biblical legal traditions that place marriage in the realm of property law. Property meaning inheritance by legitimate offspring, but property also in the sense that wives were the property of their husbands. These principles governed the understanding of adultery as based on the marital status of the woman involved. It was a problem any time a married woman slept with a man besides her husband, because it would throw the parentage of any children into doubt. It was also a problem when any non-husband slept with a married woman, because he was violating the husband’s property rights. Under late antique law like the Lex iulia (especially in its post-Christianity incarnations!), the penalty for adultery could range up to death.

You’ll notice that one category is exempt: married men could sleep with unmarried women without a formal charge of adultery getting in the way. This tradition would endure solidly into the early Middle Ages and still cling on tenaciously even later. For example, female concubines of elite men were an accepted, even expected part of court life in early medieval Italy. Legally and conceptually, they did not see this as adultery.

But an increasing force in the Middle Ages disagreed: the Church. The discomfort with the “worldly” pleasure of sex combined with its unfortunate necessity for procreation led to the Augustinian tradition that sex was licit only for reproduction and only within the confines of monogamous marriage. This represented a theoretical gender equalization in the determination of adultery, although in practice, men with power continued to flaunt what they could.

As the Church moved to increase its power (spiritual, political economic) within and over medieval society from the years around 1000 onward, sex and marriage—and sex and marriage law--became a key pillar of this control. Adultery was both a sin and a crime, subject to religious penance as well as social sanction. The Middle Ages developed two distinct legal systems, canon (Church) and secular. Adultery could actually land the accused in either court!

What constituted adultery? Well, the legal boundaries were well established: any sex that broke a marital bond. Even if the other spouse approved it: the wife would be charged as an adultress, and the husband for the separate crime of procuration or pimping (this is how the Latin legal texts are gendered), sorry, Mr. James Reynolds. In practice, adultery generally amounted to “sex beyond marriage that other people labeled adultery,” as unsatisfying as that is.

But while lords and kings continued to achieve quite the reputations for sleeping around, the Church’s hold on popular views of sex and marriage was increasingly strong. This power coincided with a growing emphasis on public order throughout society. While the internal disease of sin still mattered, the external social ramifications thereof were an increasing focus of concern. Additionally, in the 14-15C the nuclear family, with marriage at its heart, was more and more the fundamental economic unit of late medieval society. These developments combined to make adultery either the most or second most common sexual crime of the late Middle Ages! Adultery was both common and an increasing concern for people, laity and clergy alike.

The King Arthur Part

Medieval popular literature, and especially the Arthurian tradition, reflects the changing mores surrounding the need to prosecute adultery. Although the story of King Arthur himself seems to predate the 12th century, Arthurian romance with the attendant Knights of the Round Table and their paramours is a creation of the later 12C. Writers after that are indebted to the earlier, since it’s what their audiences know and expect. So different authors’ treatment of adultery is quite revealing—especially when it comes to the trio of Arthur, Guinevere, and Lancelot.

Lancelot becomes a starring figure in Arthur’s court in The Knight of the Cart by the author we call Chretien de Troyes. His love affair with Guinevere is both treason against his king and adultery, and the text even acknowledges the folly of adultery by having Lancelot defend Guinevere’s honor when she is accused of the crime (with another knight). This 12C portrayal of Lancelot is actually somewhat ambivalent on the thoroughness of his heroism.

In the 13C, a text called the Prose Lancelot shows how Lancelot in all his adulterous glory had become entrenched as a true hero in medieval readers’ minds. But when a probable-monk author got his hands on the Prose Lancelot, he wrote a sequel, Quest for the Holy Grail. In this, nobody brings out their dead and Lancelot actually does penance for his affair.

Scholars typically see this text as a direct attempt to counter a popular interpretation that deviated from social norms. And why would it not?

To use a modern parallel, the Arthur tradition is one of the Middle Ages’ greatest fanfiction products. What percentage of modern fanfic is shipfic? The characters an author wants to see together, they can finally put together in their own writing. The unconsummatable becomes consummated in later fiction, where it would not be present in the formal, official storyline.

The fraught emotions surrounding people's interpretations of love, marriage, honor, and social order combined with an increasingly rigid attitude towards Church teaching are well displayed by the 14th century master, Dante Alighieri. Awesomely, Dante even points to the troublesome role of romance literature within society! Few readers of Inferno come away without some sympathy for Francesca da Rimini, damned to the outermost circle of Hell's punishments for lust=>adultery. Her murder at the hands of her husband before she had the chance to repent seems unfair, a damnation that did not have to be.

Another Italian author Boccaccio upped the pathos of this story, inserting into the literary tradition a backstory which has Francesca tricked into a horrid marriage, presenting readers with an additional layer of tragedy.

And directly to our point here: Dante himself takes the additional step of having Francesca read the story of Lancelot as she falls into into her adulterous love! Ultimately, the Inferno must damn her for adultery, however sympathetic her plight may be...while slyly pointing out the double standard of popular romance literature.

By the time Malory is writing in the 15th century, however, concern over the social tearing of adultery, prostitution, sodomy is rising. He has 250 years of sprawling Arthuriana to squish into one text. The “facts” of the knights, including Tristam and Lancelot, are well known—this would include the adulterous affairs.

When it comes to less significant knights, Malory actually works to justify their adultery in light of honor and social order. Thus Gawain may kidnap a “knight’s lady” while he sleeps. But his heroic deed is slaying the man who slept with his (widowed) mother, since Lamorak is the son of the man who made her a widow. The death restores his family honor violated by an affair that is fornication, not adultery. By his benchmarks, Gawain remains the chivalric hero.

With Tristam and Isolde, Malory makes a crucial change to earlier tradition. The two consummate their love only after King Mark proves treacherous. Once Mark has removed himself from the plane of honor so totally, there is no honor to be violated by the love affair. Malory writes out a whole lot of “Isoldam” sex from the French tradition. In order to justify adultery to his 15C audience, Malory had to paint Mark’s actions as a worse violation of right order than adultery, in large part by cleaning up the actions of the wayward lovers.

The royal trio, however, shows most of all the increasing discomfort with adultery even in literature. Since Arthur is the faithful Christian king par excellence, Malory can’t exactly take the King Mark solution. Instead, he works from the other side. Lancelot’s presence in the Tristam section is also very carefully sanitized compared to earlier tradition: references to the physical adultery are scrubbed out. Lancelot sleeps with the unmarried Elaine, not actually Guinevere. Whenever it comes time to actually broach the question of physical passion between the storied couple directly, Malory hedges:

As the French book says, the queen and Sir Lancelot were together. And whether they were abed or otherwise, I must make no mention, for love at that time was not as love is nowdays.

Ultimately, that is where the 15th century laid their justification: love at at that time was not as love is nowadays. Acceptance of adultery in fiction did necessarily mean acceptance of adultery in reality.

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The Medieval Penitential Sex Flowchart is from James Brundage's excellent book "Law, Sex, and Christian Society in Medieval Europe." It remains one of the greatest things that medieval scholarship has ever produced and will ever produce.