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. Accordingly, it was a most disgraceful crime to produce semen and excite the woman, and to frustrate her at that very moment. He was inflamed with the basest spite and hatred. Therefore he did not allow himself to be compelled to bear that intolerable slavery. Consequently, he deserved to be killed by God. He committed an evil deed. Therefore God punished him.
[Luther's works, vol. 7 : Lectures on Genesis: Chapters 38-44 (Ge 38:9-10)]
In fact, it was believed by most of those who came before the modern age that life (ensoulment) did not even begin in the womb until "quickening" occurred (when fetal movement became detectable). I do not have the reference, but Augustine declared that abortion, though a grave sin, is not murder until after quickening. In 1140, Canon law confirmed that abortion is murder only after quickening. Indeed, for centuries, both English common law and American law (during both colonial and national periods) permitted women to have abortions until the time of quickening.
We have certainly corrected this erroneous position regarding the beginning of life, but in the opposite direction. My point is only this: that since abortion (though always considered a sin against life) was not considered by these theologians actual murder until after quickening, that likewise, contraception was not considered actual murder but a sin against life, against the wife, and, most grievously, against God, the author, creator, and sustainer of all life.