Genesis 38:9-10 (RSV) But Onan knew that the offspring would not be his; so when he went in to his brother’s wife he spilled the semen on the ground, lest he should give offspring to his brother. [10] And what he did was displeasing in the sight of the Lord, and he slew him also.

It is a matter of historical fact that no Christian communion sanctioned contraception until the Anglican Lambeth Conference in 1930. Protestant historian Roland Bainton stated that the Church “very early forbade contraception” (Early Christianity, New York: D. Van Nostrand Company, 1960, 56). The great Catholic writer and apologist Ronald Knox eloquently recounted how even the pagans detested contraception:

Ovid and Juvenal, with no flicker of Christian revelation to guide them, branded the practices in question with the protest of heathen satire. It is not Christian morality, but natural morality as hitherto conceived, that has been outraged by the change of standard. (The Belief of Catholics, 1927, Doubleday Image paperback of 1958, 31-32)

Genesis 38:9-10 (about Onan) has been one of the main proof texts traditionally used to oppose contraception. Observe how Martin Luther passionately interpreted this biblical passage:

Onan must have been a malicious and incorrigible scoundrel. This is a most disgraceful sin. It is far more atrocious than incest and adultery. We call it unchastity, yes, a Sodomitic sin. For Onan goes in to her; that is, he lies with her and copulates, and when it comes to the point of insemination, spills the semen, lest the woman conceive. Surely at such a time the order of nature established by God in procreation should be followed . . . He was inflamed with the basest spite and hatred . . . Consequently, he deserved to be killed by God. He committed an evil deed. Therefore God punished him . . . That worthless fellow . . . preferred polluting himself with a most disgraceful sin to raising up offspring for his brother. (Lectures on Genesis: Chapters 38-44; 1544; in Luther's Works, Vol. 7, 20-21)

John Calvin, in his Commentary on Genesis is no less vehemently opposed to the practice (what would he think if he knew about the vast majority of Calvinists today who regularly contracept?):

I will contend myself with briefly mentioning this, as far as the sense of shame allows to discuss it. It is a horrible thing to pour out seed besides the intercourse of man and woman. Deliberately avoiding the intercourse, so that the seed drops on the ground, is doubly horrible. For this means that one quenches the hope of his family, and kills the son, which could be expected, before he is born. This wickedness is now as severely as is possible condemned by the Spirit, through Moses, that Onan, as it were, through a violent and untimely birth, tore away the seed of his brother out the womb, and as cruel as shamefully has thrown on the earth. Moreover he thus has, as much as was in his power, tried to destroy a part of the human race. When a woman in some way drives away the seed out the womb, through aids, then this is rightly seen as an unforgivable crime. Onan was guilty of a similar crime, by defiling the earth with his seed, so that Tamar would not receive a future inheritor.

Martin Luther understood the fundamental evil of contraception and the “anti-child” mindset:

Today you find many people who do not want to have children. Moreover, this callousness and inhuman attitude, which is worse than barbarous, is met with chiefly among the nobility and princes, who often refrain from marriage for this one single reason, that they might have no offspring. . . .  Surely such men deserve that their memory be blotted out from the land of the living. Who is there who would not detest these swinish monsters? But these facts, too, serve to emphasize original sin. Otherwise we would marvel at procreation as the greatest work of God, and as a most outstanding gift we would honor it with the praises it deserves. (Lectures on Genesis: Chapters 1-5, 1536; Luther's Works, vol. 1, 118; commentary on Genesis 2:18)

The rest of the populace is more wicked than even the heathen themselves. For most married people do not desire offspring. Indeed, they turn away from it and consider it better to live without children, because they are poor and do not have the means with which to support a household. . . . But the purpose of marriage is not to have pleasure and to be idle but to procreate and bring up children, to support a household. . . . Those who have no love for children are swine, stocks, and logs unworthy of being called men and women; . . . (Lectures on Genesis: Chapters 26-30; Luther's Works, vol. 5, 325-328; vol. 28, 279; commentary on the birth of Joseph; cf. Luther's Works, vol. 45, 39-40)

But the greatest good in married life, that which makes all suffering and labor worth while, is that God grants offspring and commands that they be brought up to worship and serve him. In all the world this is the noblest and most precious work, . . . Now since we are all duty bound to suffer death, if need be, that we might bring a single soul to God, you can see how rich the estate of marriage is in good works. (The Estate of Marriage, 1522; Luther's Works, vol. 45, 46)

You will find many to whom a large number of children is unwelcome, as though marriage had been instituted only for bestial pleasures and not also for the very valuable work by which we serve God and men when we train and educate the children whom God has given us.  (In Ewald M. Plass, What Luther Says, an Anthology, two volumes, St. Louis: Concordia Publishing House, 1959, Vol. II, #2834)