What is the official teaching of the Catholic Church regarding artificial birth control?

Catholic Answer
The Catholic Church's teaching on Birth Control has been constant for twenty centuries, and is best summed up in Humanae Vitae, the encyclical letter of Pope Paul VI on Human Life. In it the Pope reiterates the Church's constant teaching in opposition to any form of artificial birth control as gravely evil.

The Church's teaching on the fecundity of marriage is covered in articles 2366-2372 of the Catechism of the Catholic Church. To sum up, it basically says that the marriage act has two essential aspects, unitive and procreative, and that anything that interfers with either of these two aspects would be a sin.
This doctrine "it is necessary that each and every marriage act remain ordered per se to the procreation of human life (Humanae Vitae 11) is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act.

In other Words, any form of birth control, outside of periodic abstience is, in and of itself, a sin; under the usual conditions it would be a mortal sin-something that would bar you from heaven.

Please note that the Church is teaching this as something received from God, and with man's final end (heaven) in view. It is not something that men, the Church, or anyone else has control over, it is just the way that God made us.