Ever hear of Onan?
It is impossible to answer your question in an FR sound byte.
The closest I can come is “it violates the natural law.”
if you are truly serious in your question, start here:
http://www.vatican.va/holy_father/paul_vi/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae_en.html
I will also point out that until the Lambeth Conference of 1930, ALL protestant denominations shared the Catholic stance on artificial birth control. When the CofE allowed it with this conference, the other protestant denominations were scandalized and reacted harshly.
This is a difficult question to answer succinctly yet charitably. For a better explanation, I refer you to Casti Connubii (Pope Pius XI, 1930), Humanae Vitae (Pope Paul VI, 1968), and Evangelium Vitae (Pope John Paul II, 1995).
Simply stated, however, the use of the condom (or any other physical or chemical contraceptive) to prevent the pregnancy of the wife intentionally separates the conjugal (baby-making) act from its procreative function. When a husband and wife engage in the conjugal act, they each sacrifice everything to the other because of their love for one another. God may choose to bless this marital love with a child. Of course, a marriage can persist and indeed thrive even if God does not bless it with children; however, why frustrate God’s designs and put you trust not in the Almighty Who can create a child for you but in some piece of plastic to thwart His blessing?
Remember, in a true marriage, marked by love and not by lust, God unites the two partners into one flesh. The condom frustrates this sacred mystery by creating an artificial barrier between the spouses. It also lowers the dignity of the wife (in the eyes of the husband) from that of a loving, nurturing mother of his children yet unborn to a mere object for his self-gratification and perverse perceived pleasures.
Applied to a different organ system, it’s akin to using a physical barrier to separate breathing from absorbing oxygen (despite no medical necessity so to do). Granted, reproduction isn’t as essential to human survival as respiration is, but the barrier compromises the reproductive system and, by extension, the marriage—perhaps imperceptibly to those involved—as surely as a piece of plastic wrap across the face compromises the respiration.
Other contraceptives effectively disable the uterus and create a hostile environment for the conceived child and lead to the premature death of an embryonic human.
But again, the popes explain far more accurately, lovingly, and powerfully than I can. So please read them.