Ask a Catholic

[The Grand Nudger]

Its against the natural law.  Recall the story of Onan?  

Heres the catechism regarding what you are discussing.  

2368  A particular aspect of this responsibility concerns the regulation of births. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood.

It isn't that the church is against controlling birth.....they are against what they deem to be immoral methods of achieving that.  The church has decided (infallibly), that methods which are deliberately contraceptive are sinful. 


Catholic Answers has this to say, as deeper commentary:

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Is contraception a modern invention? Hardly! Birth control has been around for millennia. Scrolls found in Egypt, dating to 1900 B.C., describe ancient methods of birth control that were later practiced in the Roman empire during the apostolic age. Wool that absorbed sperm, poisons that fumigated the uterus, potions, and other methods were used to prevent conception. In some centuries, even condoms were used (though made out of animal skin rather than latex). 

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). 

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Condoms are effectively, according to the church, onanism - and look at what god did to onan...... Now,that's trouble with asking a traveling apologist for an explanation for this shit. You're going to get some happy go lucky bullshit like the above rather than the raw deal..lol. God is so disgusted by onanism that he's liable to fucking kill you for it. That's why, that's it, that's all there is.