Answer: Sir Tarquine and Morgan le Fay are the characters that posed a great threat to Launcelot's honor.
Explanation:
Morgan le Fay posed a threat because it would of been against his honor to follow the lewd request. This would of made him lose personal dignity and integrity.
Sir Tarquine posed the greatest threat to Launcelot. This is because Tarquine had his brother killed by Launcelot. For this, Tarquine put Launcelot's followers/men in jail and kept them imprisoned.
