Board Thread:Questions and Answers/@comment-67.87.94.179-20121216031205/@comment-5102537-20130216102831

We don't know if he betrayed Igraine since Morgana is older than Arthur. Aside from that, adultery was punishable by death. In regard to the law and social conventions he couldn't just make an exception for himself. Moreover, if the secret had been revealed, it would have damaged Gorlois', Vivienne's, Morgana's, Arthur's and also Uther's reputation.

In real life, keeping this secret woud have been the only clever thing to do. Nowadays it might not be a problem anymore but back in those times illegitimate children were social suicide. It was dangerous and destructive to pretty much everyone involved. We should also not forget that Vivienne had at least three men. Uther, Morgause's father and Gorlois. It would have been very humiliating for her if people had learnt of that.

Plus, supposingly Uther wanted to prevent his children from fighting for the throne. Morgana was very determined, strongheaded and ambitious and Uther knew that. History is full of siblings fighting for power even by killing the opponents.