I don't have a copy of the catechism in front of me, but using birth control methods are a sin because you're going against 'be fruitful and multiply'. I don't know. That's how it used to be. Doesn't mean people listen though.
Willful exclusion of children is grounds for an anullment.
Willful exclusion of children (Canon 1101, sec. 2)Regarding natural marriage, I found this:You or your spouse married intending, either explicitly or implicitly, to deny the other's right to sexual acts open to procreation.
Traditionally speaking, the primary purpose of marriage is the generation and nurturing of offspring; the second purpose is the mutual help of spouses, and the third is the remedy for concupiscence.1 Even before marriage was perfected by grace in the Sacrament of Matrimony, God called His people to love and serve Him and each other in the married state. The intimate community of life and love which constitutes the married state has been established by the Creator and endowed by Him with its own proper laws . . . God Himself is the author of marriage. 2Marriage is the natural, indissoluble union, perfected by the Sacrament, between one man and one woman directed towards the purpose of preserving the human race by generating and raising children. Marriage is also ordered to the mutual help of spouses and the remedy for sexual desire. This definition of marriage as a natural institution can be arrived at by common sense. Nature implants in men and women an instinct that impels them to seek the companionship of marriage and in this companionship, husband and wife are able to hope for help and an easing of their physical discomforts as they get older.3