(...) if the treatment of Lewis himself had been on the whole as good as that of the other characters, and Wilson's pages on some of the books, I should have little but praise for Wilson's biography. Unfortunately, it is seriously flawed, and in one vital respect wrongheaded. This is his belief that Lewis and Joy had sexual intercourse before the Christian marriage that took place in the hospital. This is a most important matter; if it is true, it shatters Lewis's credibility as an honest man and a Christian moralist. For Lewis not only taught and believed that sexual intercourse outside marriage was utterly wrong for the Christian, he told his brother and a few of his closest friends (I had the honor to be among them) that the registry office marriage was a formality to enable Joy to stay permanently in England and that any living together as man and wife was out of the question. If at the time of such statements he was making love with Joy, he would have been a con¬temptible liar and a hypocrite. But I am sure that those who knew of the secret marriage, and indeed all his friends, had no doubt that he was an honest man who practiced what he preached.
To support his theory of Lewis's adultery, Wilson tells us that Douglas Gresham, in an oral interview taped at the Wade Center, states that in 1955, when he was only eight, he surprised his mother and Lewis in a compromising position in her bedroom. But Gresham has stated in writing that he never made this statement, and I have it on the authority of the former curator of the Wade Center, Lyle Dorsett, that Douglas's statement is not on the tape or in the typewritten transcript that was made from it.
C.S. Lewis and Adultery
George Sayer speaking of A.N. Wilson's Biography
We Remember C.S. Lewis - Essays & Memories
Broadman & Holman (2001)