User blog comment:Morganamagic7/Siblings Age/@comment-174.57.110.46-20140619152239/@comment-5674726-20140630213913

He didn't specifically say that he hadn't had sex with another woman since Ygraine died. It sounded more like he was referring to emotional companionship, that he hadn't had somebody to share his life with, in which case it is plausible that he wouldn't consider an affair with Vivienne to be any kind of substitute to the connection he shared with his wife and not something that he would speak of to Katrina.

The line was: "It's a long time since Ygraine... since anyone..." so a case could also be made that "anyone" could have been after Ygraine.

To be honest, I would say that, at the time the episode was made, the writers had not yet decided to have Morgana as Uther's daughter and, once they decided to go with that angle, they gave little, if any thought to where her conception would fit into the timeline or how to reconcile the Uther who was deeply in love with his wife with the Uther who fathered a child by the wife of his closest friend. It was a vehicle to have Morgana aim for the throne so they may have given it no more thought than "Uther's her father, Morgana wants to be Queen".