The Bible does not forbid contraception as long as it does not involve the killing of life. It is not Biblically incorrect for a married couple to decide not to have children or limit the number of children they wish to have.
People use the story of Onan as an example to support forbidding contraception; in reality these people misunderstand the context of the message. The reason God killed Onan is not because he used birth control but because his motives were to refuse to continue his deceased brother’s family line. Onan's transgression was not necessarily using sex in an un-fruitful fashion but because he refusing to allow a child of his to be connected as the seed of his brother.