User blog comment:Merlaa/Pendragons - The Diamond of the Day./@comment-5674726-20121229001036/@comment-5674726-20121229140759

Why didn't they have one?

Given the length of time they were married without a child, it seems possible, even probable, that one or both of them had a problem.

It's also possible that, due to the circumstances of Arthur's birth, he was sterile; his was a life that was brought into the world magically, and at the cost of another life. Maybe he couldn't father children because any child he fathered would upset the balance of Life and Death. Ygraine was barren, according to Nimueh, so it's not as if Arthur's children would take the places of the children Ygraine would have gone on to have, had she lived.