Matthew 1:3

Of Tamar. Tamar's name in the Old Testament (Genesis 38) is remembered for her having been twice the daughter-in-law of Judah, and later, by means of a deception, his wife also. It was with reference to her that Onan refused to raise up seed to his brother; and the Roman Catholic superstition concerning birth control is founded on this incident in the life of Onan and Tamar. Paul Blanchard's comment on this is:

Onan, not wishing to give his brother credit for paternity under the system of Jewish law, "spilled" his seed on the ground, whereupon "God slew him also." If this story has any moral, it is that all men who refuse to marry their brothers' widows should be killed. Indeed, that was the moral of the original story, since the Levirate law laid down the rule for the Jews that a man inheriting his brother's cattle and lands should also cohabit with his deceased brother's wife or wives and raise a direct heir for his brother's property. Onan's primary sin was the defiance of a property law of ancient Jews, a law that was abandoned at least 2,000 years ago! ... Catholic theologians, lacking any Scriptural authority for their extreme position on birth control, have taken this ancient story of Onan, distorted its meaning by declaring that Jehovah slew Onan for his "coitus interruptus," and inflated this "interpretation" into a whole system of social hygiene for the 20th Century.[19]