Contraception

Contraception is "any action which, either in anticipation of the conjugal act [sexual intercourse], or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible" (Humanae Vitae 14).

Contraception is evil because it is a rejection of God's gifts, because it ends life before it can even begin, and because it causes harm to the woman (sometimes permanently).  The goal of sex is to bring new life into the world.

For more information, please read this article on catholic.com.