Lewis discusses the abuse of fantasy in this excerpt from a 1956 letter.
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, 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 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…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 understand other people, (b) To respond to, and, some of us, to produce art. But is 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 (which I think bad in itself) 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.