Stephen Colbert recently explained the theological underpinnings of the Church’s opposition to contraception, and warned birth control users that their sin is a grave one:

The transmission of human life is a most serious role in which married people collaborate freely and responsibly with God. You see, to Catholics, sex isn’t two drunk strangers getting their freak on at closing time. It is the mystical union of two people inspired to create new physical life while God adds a soul in a divine and ineffably beautiful three-way. So, when you use contraception, you are not only sinning, you are cock-blocking the Almighty.

More here.