In Sir Gawain and the Green Knight, Lady Bertilak (Bercilak) is the wife of the Green Knight. She tries to seduce Gawain on her husband's behalf as he attempts to test Gawain's sense of honor. Without her presence in the story, there would have been no tale to tell, as ascertaining if Gawain is a "true and gentle knight" is the purpose of the poem.
[Lady Bertilak] comes to Gawain's bed around dawn [each morning], when Gawain is sleeping, and she plays elaborately witty games of courtship and seduction with him....she plays a quintessential role in testing Sir Gawain’s honour, loyalty, and most importantly his honesty through her sexual innuendoes, where she becomes “a potent threat to the exclusively masculine code of knightly behaviour.”
Queen Guinevere is seen in contradictory ways depending upon which version of the Arthurian legends you read. In most cases, she is tied to Lancelot, one of Arthur's most trusted knights, when they enter into an adulterous affair.
Guinevere has been portrayed as everything from a weak and opportunistic traitor to a fatally flawed but noble and virtuous gentlewoman;...[in some versions] she is praised for her intelligence, friendliness, and gentility, while in [other versions] she is a vindictive adulteress, disliked by the protagonist and all well-bred knights.
Morgan le Fay plays an important role in the Arthurian legends as well.
Early works featuring Morgan do not elaborate her character beyond her role as a fay or magician. She became much more prominent in the later cyclical prose works...
In later works, Morgan has a more prominent role in the Arthurian legends.
...in later stories she becomes an adversary of the Round Table when Guinevere discovers her adultery with one of her husband's knights, she eventually reconciles with her brother, and even retains her original role, serving as one of the four enchantresses who carry the king to Avalon after his final battle at Camlann.
Of these three women, while Lady Bertilak appears to be a seductress, she is actually loyal to her husband and only testing Gawain on her husband's behalf. Queen Guinevere, depending on the version you read, is either a woman who inspires loyalty from Arthur's knights, or strong dislike. What seems to be common in the stories is that she has an adulterous affair—in some stories, it is this that was supposed to led to the destruction of the Round Table. Morgan le Fay has the reputation of being Arthur's antagonist, but in some stories they reconcile, and she even leads him to his final resting place in Avalon. However, she is the only character whose function within the story does not revolve around an affair (or attempted affair) with a man.