There are two kinds of artificial contraception. The first is chemical, e.g. the pill. The second is what I would call "onanism," which is any attempt to prevent semen from entering a wife's uterus. This would include the use of condoms, barriers, sterilization, sodomy, and coitus interuptus.
Both kinds are evil because they artificially destroy the integrity of the nuptial embrace of husband and wife. This means that the human bodies and act of marital sex should not be tampered with in order to prevent what sex is supposed to achieve. Digesting hormones to destroy the fertility of a human being is contrary to nature. Pulling out and away from one's spouse at the moment of climax is actually greatly insulting. Introducing "devices" to prevent a full exchange is also offensive.
It's worth noting that the "pill" can and does work as an abortifacient. It changes the nature of a woman's uterus so that it is unable to receive a newly conceived human (embryo). The embryo cannot therefore attach and it dies. This is a chemical abortion and it happens all the time. Your doctor is likely to tell you that the pill is not abortive but that is because most doctors don't believe non-implanted embryos are human.
The fundamental reason that contraception is wrong is that it tampers with something that is holy. Marriage is sacred and extends back beyond the fall of man. The first commandment of God to mankind was "Be fruitful and multiply." To desire the pleasure of sex but to obstruct the fullness of the act by obstruction or drugs is disgraceful. Can you imagine receiving Communion but also doing everything in your power to prevent the Eucharist's purpose and power? I believe we call such an action "sacrilege."
If you are contracepting, I would encourage you to check out the Couple to Couple League. They are very helpful.