From a letter by CS Lewis to Mr. Masson dated March 6th, 1956 found in the Wade Collection at
“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. 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 understand other people, (b) to respond to, and, some of us, to produce art. But it also has 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 is 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 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."
Another thing that I think worth noting is the causal effect that can be drawn between our imagination and reality. Now, I have taken classes in which we were told that we can and indeed it is good to imagine everything we want. The stated reason for this is that we can repress important emotions and build up tensions that result in horrible breakouts of anger, violence, and what have you. I disagree with this on principle that, as CS Lewis shows, the real danger of masturbation is that the desire for attachment (which is the deepest seated of out desires) does not accept substitutes that our imagination creates. Thus by supplanting the real needs of our psyche with imaginary ones and wrapping ourselves up in this harem that we have created we increase the probability of a violent release of this tension.Not only this, but the objectification of another human being inherent in the imaginary "harem" while is not necessarily causally related cannot be supposed to have no relation to one's actions and predispositions towards actual human beings. If I am constantly having these imaginary women always at my beck and call, why won't these thoughts slip into my actual life?
It smacks of the situation of Tantalus from the view of the person who is interacting with a person who masturbates because it would be attempting but always failing at meeting the person's needs. It would be impossible (even if it was possible it would not be good) to live up to the standards of the person who masturbates because one could not emulate the imaginary harem.