From: The Collected Letters of C. S. Lewis, Vol. II: Books, Broadcasts, and the War, 1931-1949, edited by Walter Hooper, HarperSanFrancisco, 2004:
The three ‘reasons’ for marrying, in modern English are (a) To have children. (b) Because you are very unlikely to succeed in leading a life of total sexual abstinence, and marriage is the only innocent outlet, (c) To be in a partnership. What is there to object to in the order in which they are put? . . .
[editor Walter Hooper added in a footnote: “Lewis is citing the service for the ‘Solemnization of Matrimony’ in the Book of Common Prayer: ‘First, It was ordained for the procreation of children . . . Secondly, It was ordained for a remedy against sin, and to avoid fornication . . . Thirdly, It was ordained for the mutual society, help, and comfort, that the one ought to have of the other.'”]
The Prayer Book therefore begins with something universal and solid — the biological aspect. No one is going to deny that the biological end of the sexual function is offspring. And this is, on any sane view, of more importance than the feelings of the parents. Your descendants may be alive a million years hence and may number tens of thousands. In this regard marriages are the fountains of History. Surely to put the mere emotional aspects first would be sheer sentimentalism. (18 April 1940, pp. 392-293)
As a bachelor I think I should be imprudent in attacking it [contraception]: on the other hand I should not like the job of defending it against the almost unbroken Christian disapproval. (19 August 1947, p. 798)
From: The Abolition of Man (New York: Macmillan, 1947):
As regards contraceptives, there is a paradoxical, negative sense in which all possible future generations are the patients or subjects of a power wielded by those already alive. By contraception simply, they are denied existence; by contraception used as a means of selective breeding, they are, without their concurring voice, made to be what one generation, for its own reasons, may choose to prefer. From this point of view, what we call Man’s power over Nature turns out to be a power exercised by some men over other men with Nature as its instrument. (pp. 68-69)
From: The Collected Letters of C. S. Lewis, Vol. III: Narnia, Cambridge, and Joy 1950-1963, edited by Walter Hooper, HarperSanFrancisco, 2007:
It certainly seems very hard that you should be told to arm the young against Venus without calling in Christ. . . . [N]ow that contraceptives have removed the more disastrous consequence for girls, and medicine has largely defeated the worst horrors of syphilis, what argument against promiscuity is there left which will influence the young unless one brings in the whole supernatural and sacramental view of man? (28 April 1955, p. 600)
Christians . . . of course agree that man & wife are ‘one flesh’ . . . this One Flesh must not (and in the long run cannot) ‘live to itself’ any more than the single individual. It was not made, any more than he, to be its own End. It was made for God and (in Him) for its neighbours — first and foremost among them the children it ought to have produced. (The idea behind your voluntary sterility, that an experience, e.g., maternity, which cannot be shared should on that account be avoided, is surely very unsound. For a. (forgive me) the conjugal act itself depends on opposite & reciprocal and therefore unshare-able experiences. . . .) (8 May 1955, pp. 605-606)
Bible vs. Contraception: Onan’s Sin and Punishment [National Catholic Register, 5-30-17]
Dialogue w Several Non-Catholics on Contraception [1996 and 1998]Contraception: Early Church Teaching (William Klimon) 
Luther and Calvin Opposed Contraception and “Fewer Children is Better” Thinking [2-21-04; published at National Catholic Register, 9-13-17]
Contraception and “Anti-Procreation” vs. Scripture [National Catholic Register, 6-6-18]
A Defense of Natural Family Planning [National Catholic Register, 5-25-19]
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