'Listen, suppose your time were come to die, And you were quite alone and very weak; Yea, laid a dying while very mightily The wind was ruffling up the narrow streak Of river through your broad lands running well [...]' (lines 16-20)
These are Guenevere's first words in her defense, after her initial apology. She begins abruptly: "Listen!" (as though these guys could help listening to her). But instead of launching into a string of excuses for her behavior with Launcelot, or a lengthy explanation of why things looked the way they did, she asks them to imagine that they're on their deathbed. Interesting choice.
'Suppose a hush should come, then some one speak: "One of these cloths is heaven, and one is hell, Now choose one cloth forever, which they be, I will not tell you, you must somehow tell Of your own strength and mightiness; here, see!"' (lines 21-25)
Now Guenevere comes to her point: she asks her listeners to imagine that they're offered an impossible choice on their deathbed between heaven and hell, with no good way of telling the difference between the two. That's a heck of a choice to ask a dying person to make.
'After a shivering half-hour you said, "God help! heaven's colour, the blue;" and he said, "hell." (lines 37-38)
Guenevere imagines that these knights would choose the cloth that seemed most likely to get them into heaven – the blue one – since, after all, the sky is blue and hellfire is red. But blue is wrong! How could they have guessed? What is Guenevere's point in all of this? Why does she ask the knights in the courtroom to imagine themselves faced with an impossible choice?
'Behold my judges, then the cloths were brought: While I was dizzied thus [...]' (lines 80-81)
This is where Guenevere kind of explains why she introduced that long <strong>allegory</strong> about the deathbed choice between the red and blue cloths. After Launcelot came to Arthur's court, she got all confused and "dizzied" with memories of the past and had to make a decision. Like in the "what-if" situation she describes with the "red" and "blue cloths," she had no good way of telling which choice was right, and no one to consult but herself.
'[...] must I now prove Stone-cold for ever? Pray you, does the Lord Will that all folks should be quite happy and good?' (lines 88-89)
Here Guenevere offers her listeners a bit of the reasoning she used to make her choice. For her, it was a choice between being miserable, "stone-cold forever" and being "happy and good." You can guess which choice she made.