My husband and I are going to sign up for NFP, however I have a question about contraception. I know that the church teaches that all forms of contraception are wrong because it prevents pregnancy.
This is not why contraception is gravely immoral. Contraception separates the unitive and procreative elements of the marital act. God ordered our sexuality in a particular way. Whenever we choose to engage in the marital embrace, we must maintain the elements in the way God designed them. We may not separate them.
We can chose when and whether we engage in the marital embrace, but when we choose to do so we may not separate the unitive and procreative elements in any way.
The Bible says to be fruitful and multiply, and that is why couples should have children. However, why does this not apply to priests as well?
God gave this command to Adam and Eve within the context of marriage.
We are each called to chastity in our state in life. For priests and other unmarried people this means refraining from sexual intimacy. Only within marriage does God’s command regarding fecundy apply.
The Bible doesn’t command all people to get married, so does it command all ‘married’ people to reproduce?
No, it is not a requirement-- nor even a possibility-- that each married couple reproduce. It is, in general, the purpose of marriage and part of the consent required when we marry. The threefold elements of marriage are permanency, fidelity, and fecundy. Some enter marriage intending to conceive but may then encounter circumstances that prevent it.
Why should the ‘be fruitful and multiply’ rule only apply to those who get married, or better yet why does it not mean that all people are supposed to get married and multiply?
The use of our sexual faculties is for the purpose of procreation, and that is always within marriage.
Not all people are called to marriage-- we know this because Christ himself tells us that some will be celibate for the Kingdom. If God’s command given to Adam and Eve was a requirement of every person, Christ’s teaching would have been radically different than what he actually did teach. He reaffirmed the command regarding the two becoming one in marriage. He did not indicate that all must marry. He indicated the opposite-- some are called to celibacy for specific purposes.
I suggest you read the Catechism sections on the Sacrament of Marriage and on the Sixth Commandment as well as the Papal Encyclical Casti Connubii.