Any Catholic who cites the example of Onan as an authoritative statement re contraception should do a study on the "kinsman redeemer" laws in the Old Testament (cf. Deut. 25:5-6). Onan wasn't guilty for ejaculating outside of a woman, he was guilty of willfully trying to utterly disinherit his brother's family from Israel (IIRC the entire Tribe of Judah, as Onan had no children of his own). Any other reading of the text is, IMO, a false exegesis.
"Then Judah said to Onan, "Go in to your brother's wife, and perform your duty as a brother-in-law to her, and raise up offspring for your brother. Onan knew that the offspring would not be his; so when he went in to his brother's wife, he wasted his seed on the ground in order not to give offspring to his brother." (Genesis 38:8-9)If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her. And it shall be, that the firstborn which she beareth shall succeed in the name of his brother which is dead, that his name be not put out of Israel (Deut. 25:5-6).
Exactly and excellent reference.
FR thread: Protestants and Birth Control.
You can follow that link to find the usual excerpts from Luther, Calvin, Wesley, etc., who interpreted the Onan incident as showing God's detestation of what he did: having intercourse while deliberately frustrating the fertility thereof. This, they say, God found morally offensive precisely on those grounds. As well, these eminent men of the Reformation cited this lesson as being applicable to their own Christian contemporaries, to whom the levirate obligation would be inapplicable.
So my first questions are: why do you think these reformers thought that the Onan incident was a moral lesson for the present (their own Christian communities)? Was it because they believed that Christians also have a levirate obligation? If not, then was it because they found the choice of sabotaging the natural fertility of intercourse morally objectionable?
It is only recently (since, say, the 19th century) that anyone started making the novel argument that Onan's sin was simply, and only that he refused his obligation in the custom of the time, which was to provide his brother's widow with a child. Why does this argument seem, even at first glance, insupportable? Because the biblical penalty for not giving your brother’s widow children was public humiliation, not death (Deuteronomy 25:7–10). Not only that, but Onan's father, Judah, also refused to impregnate the widow Tamar (he refused to have intercourse with her) --- but he was not struck dead by God.
This shows that Onan's sin was more than merely not fulfilling the duty of a male next-of-kin; his sin was the sacrilege of "going through the motions" of real intercourse, while deliberately perverting it so it would be infertile.
This is the conclusion that was reached by the Jewish Talmud, by early Christian leaders such as St. Jerome and St. Clement of Alexandria, and by all of the Protestant Reformers (Luther, Calvin, etc.) In fact, it's what all Christians agreed for almost 2,000 years, before the pro-contraceptive Anglican Lambeth Conference of 1930, which broke with all previous Christians in calling evil, good.
I can't find any writer of the Lutheran, Calvinist, Reformed, Puritan, Baptist, Anabaptist, or any other Christian before 1900 who wrote that deliberately nullifying the natural fertility of intercourse was a morally upright thing to do.
So my next question is: can you?