The first of several voices from church history, from Martin Luther's Lectures on Genesis, on Genesis 38:
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 preferred polluting himself with a most disgraceful sin to raising up offspring for his brother.
As found on BaylyBlog.