Martin Luther said, "[T]he exceedingly foul deed of Onan, the basest of wretches . . . 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 sperm, 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. . . . Consequently, he deserved to be killed by God. He committed an evil deed. Therefore, God punished him."
John Calvin said, "The voluntary spilling of sperm outside of intercourse between man and woman is a monstrous thing. Deliberately to withdraw from coitus in order that sperm may fall on the ground is doubly monstrous. For this is to extinguish the hope of the race and to kill before he is born the hoped-for offspring."
John Wesley warned, "Those sins that dishonor the body are very displeasing to God, and the evidence of vile affections. Observe, the thing which he [Onan] did displeased the Lordand it is to be feared; thousands, especially of single persons, by this very thing, still displease the Lord, and destroy their own souls." (These passages are cited in Charles D. Provan, The Bible and Birth Control, which contains many quotes by historic Protestant figures who recognize contraceptions evils.)
Above from
http://www.catholic.com/library/birth_control.asp
"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 or women; for they despise the blessings of God, the Creator and Author of marriage." ---Martin Luther
Synod of Dort, a Calvinist council, declared,
[Onans contraceptive act] was even as much as if he had, in a manner, pulled forth fruit out of the mothers womb and destroyed it.
Above copied from:
http://catholicoutlook.com/objcath5.html
Hope it helped,
Neal