The traditional treatment of women in the Arthurian saga is interesting to note in its severity. Guinevere is an adulteress, Morgan Le Fay is a witch, Morgause is an incestuous schemer, and the Lady of the Lake is the leader of a terrible, backward pagan religion. This is, of course, the treatment that has been handed down to us from the medieval writers who gave us the Christian infusion that spawned the story of the Holy Grail.
But if we go back to the historical writers——the writers who were treating Arthur as a historical character, not a target for pious retribution or moral sermonizing; we find that women do, more or less, have quite an important role to play in the overall health and well-being of Arthur the King.
Inherent in this last statement is this feature of earlier stories: Arthur's story did not end badly. In the earliest of stories about Arthur the King, he wasn't even the Once and Future King. He was a great warrior and won many great victories. He was a great king who had a great queen, and they both ruled a great kingdom.
Arthur's queen didn't even have a name in the earliest of stories. She was simply the queen. But she didn't, as so many modern stories tell us, have a part in the downfall of the kingdom. Even Geoffrey of Monmouth, in the happier parts of his story, has Arthur and Ganhumara (his name for the queen) holding court in a great castle in a great city called the City of the Legions. (Geoffrey later gives Guinevere a bad name, but she is all that glitters when the story opens.)
Also showered with the light of good in earlier stories is Morgan Le Fay. In the earliest of the Cornwall stories, Morgan is the Queen of Lyonesse, where Arthur is taken after his final battle. Morgan is the leader of a group of healers, all women, who will undoubtedly heal Arthur. It is perhaps out of this part of the story that William of Malmesbury crafted his vision that Arthur would come again. (William, you will remember, is the first one to mention that Arthur's grave has not been found; even though he says nothing more, he gives a strong hint that the Once and Future King part of the legend has begun.)
The Lady of the Lake as well fares rather better in the older stories. She it was who gave Arthur his first sword. She it is who is guardian of the old ways, the ways from which comes much of the magic of the original Arthurian stories. The Christian writers wanted to make Arthur a Christian king, and they didn't want any other religions getting in the way of this. Naturally, they made the Lady of the Lake a questionable character solely because of her "religious" ways.
The question then becomes one of why. Why do the women come off so bad in medieval stories? What happened to make the writers change their image of people who were so vital to the story? The answer probably lies in the spiritual realm and its resulting scriptures. The same writers who were reading the Bible and discovering that Eve was responsible for all the world's problems probably thought that they should blame Guinevere for all of Arthur's problems. Why did Arthur have a bastard son in the first place? Because the scheming Morgause tricked him into it. Why did Morgan Le Fay hate Arthur so? Because she was jealous of the magic wielded by Merlin, Arthur's advisor. Why was the Lady of the Lake suddenly to be reviled, not revered? Because she was the head of a religion that medieval writers didn't understand or want to understand. The teachings of the Bible were such that one religion was possible (in the minds of these writers); anything else was unacceptable. (It must be said here that Arthur, Lancelot, and all the rest of the male characters take quite a beating at the hands of the medievalists as well. The whole story becomes a morality play from which only Galahad, the purse and chaste, emerges unscathed. Arthur and Lancelot, the perfect king and the perfect knight, are undone by their shared imperfect morality.)
The result is that in the modern tales, women are still pretty much blamed for Arthur's troubles. Arthur's dallying with Morgause has come to be seen as something akin to the Adam and Eve story. Lancelot's love for the queen has come to be seen as equal in its devastation to the queen's own adulterous actions (that is to say, it's the woman's fault). Morgan Le Fay is reviled as a witch who wants nothing more than to ruin the kingdom of the wonderful Arthur. And the Lady of the Lake recedes ever further into the background. (Some traditions hold that she is Nimue, whose claim to "fame" is that she enchants Merlin into submission and takes him from Arthur when Arthur needs him most.)
If you want to role-play in Arthur's world, you'd better choose a male character. At least people will feel sorry for you.