I came to the conclusion that it was more a film of/about the beheading-contest tale-type than an adaptation of your actual Cotton Nero A.x., though I do think in some sense it is about chastity in the sense 'sexual ethics' too. I read the closing vision as a cautionary tale about the dangers of heterosexuality patriarchy? But I don't think it gets close to being as queer as the poem.

What I really missed was the poem's vivid sense of warm hospitable interiors opposed to the bleak and cruel wilderness outdoors, which I think a cinematic treatment could have done so much with! But it would require medieval people to appear clean, happy and brightly-dressed, a thing no director has seemed willing to depict these 30 years or more. Bertilak's castle looked like the worst New England B&B in existence.