Someone asked me 'Can you tell me why Catholics are not allowed to use condoms as a form of contraception?'  Although the question was about condoms in particular, the reasoning applies to other contraceptives as well.  The following is the reply I gave.

To explain why Catholics cannot use condoms as a contraceptive I will have to outline the Catholic understanding of God as Creator and the Catholic understanding of marriage.  I will be as brief as I can.

The Catholic Church believes that God is Creator and that God created man, male and female He created them, and that God created them "in His own Image".  Created in God’s Image, human life is sacred.  In marriage, a man and a woman cling to each other and become one flesh.  In marriage the marriage act (sexual intercourse) is a participation in God’s creative act, the act of creating a sacred human life.  God has created each human life for and with a purpose - to love and to worship God and to live for Him, and with Him, for eternity.

The Church teaches that the marriage act (sexual intercourse), for it to be truly and authentically "marital", must be both "procreative" and "unitive".  Procreative in this sense means "being open to the creation of human life" and unitive in this sense means that the "two (a man and a woman) become "one" in a total gift of self to the other".  Each and every "marriage act" must be both "open to the creation of human life" and must also be "unitive".  This is God’s intention, design and purpose for the marriage act.  In marriage the two (a man and a woman) are united in Christ.

To remove from the marriage act either the procreative meaning of the act and/or the unitive meaning of the act, violates God’s intention, design and purpose of the marriage act (sexual intercourse).  This explains why sexual acts outside marriage are sinful; they violate God’s intention, design and purpose for the marriage act, and that, within marriage itself, only chaste, pure, procreative and unitive sexual intercourse make the sexual act truly and authentically marital.

Condoms, by their contraceptive nature and action, violate the procreative meaning as well as the unitive meaning of the marriage act.  The use of a condom in the marriage act means that the man can not give all himself to the woman and the woman can not give all herself to the man.  They withhold not only something of themselves from each other, and so fall short of the total giving required of the marriage act to be marital so that the two cannot become "one", but also condom use in the marriage act opposes procreation.

The word "contraception" means "against conception".  Contraception, being against conception, violates a fundamental purpose of God’s creative plan for the marriage act.  The Church’s teaching follows the same reasoning for other forms of contraception, not just condoms, although many other forms of contraception can also be abortifacient (abortion causing) - by preventing the newly conceived human life from implanting in its mother’s womb, which is an added objection to their contraceptive nature.