And well…considering how Kovu looks, and the fact that his mother, Zira, is implied to be Scar’s queen, and the fact that Kovu is a member of the Outlanders, a pride of “evil” lions who believe Scar was/is the rightful king and Simba was a usurper…people took that and ran with it.

And they ran straight into incest jokes. Since Kiara IS Simba’s daughter and IF Kovu is Scar’s son, then the two of them would be first cousins once removed.