What is the controversy over Arthur's birth in The Coming of the King from Idylls of the King?

Many claim that Arthur is not King Uther's son, but an impostor who was born to ignoble parents. Arthur fights off the heathen invaders and pushes back the beasts, restoring order to Leodogran's kingdom. As a reward for his service, he asks for Guinevere as his wife, but Leodogran is apprehensive because of the rumors of Arthur's low birth.