A 1956 letter responding to Keith Masson, a young American.
Dear Mr Masson -
. . . . I agree that the stuff about 'wastage of vital fluids' is rubbish. 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: send the man back into the prison of himself, there to keep a harem of imaginary brides. And this harem, once admitted, works against his ever getting out and really uniting with a real woman. For the harem is always accessible, always subservient, calls for no sacrifices or adjustments, and can be endowed with erotic and psychological attractions which no real woman can rival. Among those shadowy brides he is always adored, always the perfect lover: no demand is made on his unselfishness, no mortification ever imposed on his vanity.
In the end, they become merely the medium through which he increasingly adores himself. Do read Charles Williams' Descent into Hell and study the character of Mr. Wentworth. And it is not only the faculty of love which is thus sterilized, forced back on itself, but also the faculty of imagination.
The true exercise of imagination, in my view, is (a) To help us to understand other people (b) To respond to, and, some of us, to produce, art. But it has also a bad use: to provide for us, in shadowy form, a substitute for virtues, successes, distinctions etc. which ought to be sought outside in the real world--e.g. picturing all I'd do if I were rich instead of earning and saving. Masturbation involves this abuse of imagination in erotic matters . . . and thereby encourages a similar abuse of it in all spheres. After all, almost the main work of life is to come out of our selves, out of the little, dark prison we are all born in. Masturbation is to be avoided as all things are to be avoided which retard this process. The danger is that of coming to love the prison.
C. S. Lewis
--Collected Letters of C. S. Lewis: Vol 3 (Cambridge University Press, 2007), 758-59; emphasis original