And God blessed them; and God said to them, "Be fruitful and multiply, and fill the earth, and subdue it; and rule over the fish of the sea and over the birds of the sky, and over every living thing that moves on the earth." (NAS, Genesis 1:28)
The second passage has to do with the Old Testament law of levirate marriage (Deuteronomy 25:5-10). If a man died childless, his brother or nearest relative was expected to marry his widow and father a child to carry on the deceased's name and inherit his property. (These Old Testament laws do not apply to Christians.) Onan refused to fulfill that duty and used a birth control method known as coitus interruptus to prevent pregnancy:
Then Judah said to Onan, "Lie with your brother's wife and fulfill your duty to her as a brother-in-law to produce offspring for your brother." But Onan knew that the offspring would not be his; so whenever he lay with his brother's wife, he spilled his semen on the ground to keep from producing offspring for his brother. What he did was wicked in the LORD's sight; so he put him to death also. (NIV, Genesis 38:8-10)
Most experts say Onan was condemned for using his brother's widow for sexual pleasure while refusing to provide offspring for him, and that no general criticism of birth control was intended.
The Bible gives clear, direct guidance on many topics of morality, but not on birth control. Thus, any inferences from the Bible are opinions and not Biblical evidence.
At one time it was believed the male sperm was the only factor in human reproduction and the sperm was regarded as humans in miniature. Hence spilling it out was tantamount to abortion as well as a waste of a precious element.1
It is now known that sperm cells are not miniature humans; a man's sperm must unite with a woman's egg before a baby can be formed. Furthermore, sperm cells not ejected from the body simply die after a few weeks anyway, and they are continuously replaced.
In the twentieth century, scientific knowledge of reproduction, sensitivity to women's rights, and concern about overpopulation produced great changes in attitude. Most Christian churches now say reproductive decisions are private matters between husband and wife and their consciences. Birth control methods are no longer discouraged.
The Roman Catholic Church is a notable exception. The official church teaching still opposes all forms of birth control except abstinence and the rhythm method, saying, "it is necessary that each and every marriage act remain ordered per se to the procreation of human life.2" However, polls show that only 15% of U.S. Catholics view contraception as morally wrong.3
Although attitudes about contraception have changed, many Christians and many churches continue to oppose abortion as a means of birth control.
1Thomas Bokenkotter, Essential Catholicism, Doubleday, 1985, p. 334.
2United States Catholic Conference, Inc., Catechism of the Catholic Church, second edition, 1994, ¶ 2366
3Pew Research Center, http://www.pewresearch.org, poll data from February, 2012.