“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 grandchildren) and turns it back; send the man back into 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, [that is, the fantasy women in his imagination] the harem is always accessible, always subservient, calls for no sacrifices or adjustments, and can be endowed with erotic and psychological attractions which no 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.…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.”
COLLECTED LETTERS OF C. S. LEWIS: Vol. 3; (Oxford University Press, 2007); pp. 758-759