For me the real evil of masturbation would be that it takes an appetite which, in lawful use, leads the individual out of himself to complete (and correct) his own personality in that of another (and finally in children and even grandchildren) and turns it back; sends the man back into the prison of himself, there to keep a harem of imaginary brides.
And this harem, once admitted, …
The Collected Letters of C.S. Lewis v. 3, Personal Letter From Lewis to Keith Masson, by C.S. Lewis