>>It is not mentioned, but it can be reasoned out with reference to the goodness of God's design of male and female, and the wrongness of altering and rearranging this design at will.<
Then do that. But don't declare something not clearly defined in scripture on the misapplication of one verse.
It's not a misapplication, it's how the verses were read in every generation before the 20th century. Then, when people wanted to use contraceptives, the interpretation changed. Imagine that.
A good summary
"That Onan's unnatural act as such is condemned as sinful in Gen. 38: 9-10 was an interpretation held by the Fathers and Doctors of the Catholic Church, by the Protestant Reformers, and by nearly all celibate and married theologians of all Christian denominations until the early years of this century, when some exegetes began to approach the text with preconceptions deriving from the sexual decadence of modern Western culture and its exaggerated concern for 'over-population.' "