Is it true that in some versions of the legend, Vortigern has a sexual affair with his own daughter, who agrees to marry him in order to become Queen? Does this happen before or after Vortigern marries the Saxon beauty, Lady Rowenna?
I have to admit that while I read Nennius I completely forgot about it, as tend to be less interested to Vortigern and Uther, but, here it is, looking it up, it looks like this comes from the History of Britons by Nennius:
39. In the meantime, Vortigern, as if desirous of adding to the evils he had already occasioned, married his own daughter, by whom he had a son. When this was made known to St. Germanus, he came, with all the British clergy, to reprove him: and whilst a numerous assembly of the ecclesiastics and laity were in consultation, the weak king ordered his daughter to appear before them, and in the presence of all to present her son to St. Germanus, and declare that he was the father of the child. The immodest* woman obeyed; and St. Germanus, taking the child, said, “I will be a father to you, my son; nor will I dismiss you till a razor, scissors, and comb, are given to me, and it is allowed you to give them to your carnal father.” The child obeyed St. Germanus, and going to his father Vortigern, said to him, “Thou art my father; shave and cut the hair of my head.” The king blushed, and was silent; and, without replying to the child, arose in great anger, and fled from the presence of St. Germanus, execrated and condemned by the whole synod.