It should be banned, as it does no favors to the gene pool. However, the uncomfortable truth is that, go back 4, 5, 6, 7+ generations, and likely every single person in this thread has ancestors who bred with first cousins.
And maybe even siblings. Go back far enough, we probably all have an ancestor who was conceived during a r*pe. Human history is often ugly.
When it happens rarely, its not a problem. When it happens regularly you get Hapsburg Jaws, and other similar problems.