It is interesting that you mentioned 1 Corinthians 6:12 as showing that contraception is allowable, presumably since it falls under the category of the word “all” as in “
all things are lawful to me.” What must be understood, however, is that St. Paul here makes a comparison between those under the law and those outside of the law (the Jews and the Christians.) The distinction is between those that are under the Law of Moses and those that are under the law of faith. If one is to say that contraception is allowed under this passage, then one must not forget the rest of the passage: “
all things are not expedient” (1 Cor. 6:12). After all, in the very next verse, St. Paul tells us that “
the body is not for fornication, but for the Lord, and the Lord for the body” (1 Cor. 6:13).
Your interpretation of the passages of Genesis 38 is partially correct, though very much incomplete. Catholic Answers has a tract on this very issue and here’s what the apologists say:
Catholic Answers Faith Tracts on Contraception
The Bible mentions at least one form of contraception specifically and condemns it. Coitus interruptus, was used by Onan to avoid fulfilling his duty according to the ancient Jewish law of fathering children for one’s dead brother. “Judah said to Onan, ‘Go in to your brother’s wife, and perform the duty of a brother-in-law to her, and raise up offspring for your brother.’ But Onan knew that the offspring would not be his; so when he went in to his brother’s wife he spilled the semen on the ground, lest he should give offspring to his brother. And what he did was displeasing in the sight of the Lord, and he slew him also” (Gen. 38:8–10).
The biblical penalty for not giving your brother’s widow children was public humiliation, not death (Deut. 25:7–10). But Onan received death as punishment for his crime. This means his crime was more than simply not fulfilling the duty of a brother-in-law. He lost his life because he violated natural law, as Jewish and Christian commentators have always understood. For this reason, certain forms of contraception have historically been known as “Onanism,” after the man who practiced it, just as homosexuality has historically been known as “Sodomy,” after the men of Sodom, who practiced that vice (cf. Gen. 19).
catholic.com/library/Birth_Control.asp
The early Church has always believed that the improper use of the seed (Onanism) is very much contrary to the natural law of God.
Clement of Alexandria
*“Because of its divine institution for the propagation of man, the seed is not to be vainly ejaculated, nor is it to be damaged, nor is it to be wasted” (The Instructor of Children 2:10:91:2 [A.D. 191]).
“To have coitus other than to procreate children is to do injury to nature” (ibid., 2:10:95:3).*
Lactantius
“God gave us eyes not to see and desire pleasure, but to see acts to be performed for the needs of life; so too, the genital ’generating’] part of the body, as the name itself teaches, has been received by us for no other purpose than the generation of offspring” (ibid., 6:23:18) [A.D. 307]).
Council of Nicaea I
"f anyone in sound health has castrated himself, it behooves that such a one, if enrolled among the clergy, should cease [from his ministry], and that from henceforth no such person should be promoted. (Canon 1 [A.D. 325]).
Augustine
"I am supposing, then, although you are not lying [with your wife] for the sake of procreating offspring, you are not for the sake of lust obstructing their procreation by an evil prayer or an evil deed. Those who do this, although they are called husband and wife, are not; nor do they retain any reality of marriage, but with a respectable name cover a shame. (Marriage and Concupiscence 1:15:17 [A.D. 419]).
Jerome
“But I wonder why he [the heretic Jovinianus] set Judah and Tamar before us for an example, unless perchance even harlots give him pleasure; or Onan, who was slain because he grudged his brother seed. Does he imagine that we approve of any sexual intercourse except for the procreation of children?” (Against Jovinian 1:19 [A.D. 393]).
John Chrysostom, St
"Why do you sow where the field is eager to destroy the fruit, where there are medicines of sterility [oral contraceptives], where there is murder before birth? You do not even let a harlot remain only a harlot, but you make her a murderess as well. . . . Indeed, it is something worse than murder, and I do not know what to call it; for she does not kill what is formed but prevents its formation. What then? Do you condemn the gift of God and fight with his [natural] laws? (Homilies on Romans 24 [A.D. 391]).
What a low view of sex these people have then! For them and the RCC, the purpose of sex is merely to procreate (to propagate the species), while most Protestants, gleaning from the teachings of the Bible, say that sex within the confines of marriage, have a two-fold purpose, i.e. 1) as an expression of marital love and passion, and 2) as an act of procreation. Absent any one of the two purposes of sex would make man nothing more than an animal with regard to sexual activity.
The couple decides. They have brains and human will, God gave us that. Nobody forces you to contracept, it woould still depend on the choice of the couple whether or not to plan their family size, to use whatever methods of contraception, whether NFP or artificial means, provided it is not abortion.