'God wot I ought to say, I have done ill, And pray you all forgiveness heartily!' (lines 13-14)
At the beginning of the poem Guenevere seems to be acknowledging that most people would think she has "done ill" (done wrong), but she avoids using the word "sin." ("Wot," by the way, is an archaic word for "know," so she's saying "God knows I ought to say ...") You could translate what she's saying as something like, "God knows I should just admit that I've sinned and ask your forgiveness."
'After a shivering half-hour you said, "God help! heaven's colour, the blue;" and he said, "hell." Perhaps you then would roll upon your bed, And cry to all good men that loved you well, "Ah Christ! if only I had known, known, known;"' (lines 37-41)
Guenevere asks her listeners to imagine that they have to choose between a red and a blue cloth – one of which represents heaven, and one hell – and they have no way of knowing which is which. She compares her own decisions to this arbitrary choice. Is the definition of "sin" really that arbitrary in the world of this poem? Guenevere certainly wants to persuade her jury that it is.
'And felt strange new joy as the worn head lay Back, with the hair like sea-weed; yea all past Sweat of the forehead, dryness of the lips, Washed utterly out by the dear waves o'ercast In the lone sea [...]' (lines 99-103)
Guenevere imagines lying back at the seashore with her hair in the ocean waves. The image of the sea washing "sweat" and "dryness" from her could be a <strong>metaphor</strong> for the waves washing away her sin, but it's not really clear. She persists in avoiding the use of the word "sin" at all.
'When you can see my face with no lie there For ever? [...]' (lines 240-241)
Guenevere uses her own beauty as proof that she has not sinned, or "lie[d]." Yet again, she avoids using the "s" word and instead talks about lying. Is she implying that lying is a greater sin than adultery? Or is Guenevere maybe trying to divert the jurors' attention away from the issue of adultery by focusing on the lying?