Not all contraception is a sin by itself. There are two cases where using contraception is a sin. In one, it's a sin if it kills the already fertilized egg, as he's a person already. In the other, it's a sin if it aid fornication: That is, the abuse of sex for own pleasure. Take into account that masturbation and pornography also fall in this category.

There are exceptions in the first category, however. Say, a woman who is sexually inactive could use a non-abortive (prevents fertilization only, allows for implantation and growth once the egg is fertilized) contraception if she is in fear of being raped. But there are rules in this case: As I said, the contraceptive must be non-abortive, and if she ends up pregnant, she has to respect her child's right to life and carry to term. She can give him for adoption if she doesn't want to raise him, or she can keep him. This way, everyone's rights are respected, no one sins.

It's complicated, so it's better not to use contraceptives at all and just abstain from fornication.