Now to be completely fair Guinevere was actually an incredibly faithful wife until a Frenchman's 'totally not a self insert original character fan fiction' got confused with legitimate Arthurian canon. Unfortunately that confusion happened at a critical juncture, and the Frenchman's OC got carried over into every later iteration of Arthur's story.

Also, Morgan le Fay's villainy is something that got inflated over the centuries of retellings and when you go back to when the Frenchman's OC was introduced only 5% of her plots were to take over the Kingdom, 45% were attempt's to expose Guinevere's affair with the Frenchman's OC and the remaining 50% were convoluted plots to kill Guinevere. Basically Morgan wasn't Arthur's nemesis, but Guinevere's. In the older stories Morgan actually does a lot of good things for Arthur, and after the final battle it's Morgan who desperately labors to keep Arthur alive long enough to be taken to Avalon where he can be healed.

It's actually kind of funny that the various retellings of Arthur's story slowly turned all the women around Arthur into such horrible matches that an amorous squirrel seems like the obvious choice for best girl.