There are not many issues which stir as much controversy as birth control among devout Christians. The Catholic Church has taught against it forever, and its teachings were fleshed out in the relatively recent encyclical Humanae Vitae. Increasingly, evangelicals of other denominations have begun to see the merit of not practicing birth control.
For my own part, I have not made up my mind 100%. Certainly, scriptural teaching leaves the issue somewhat open. There is no direct condemnation of family planning. However, there is a philosophical argument based off the Bible for opposing birth control.
The question we need to ask is why was marriage instituted in the first place.
"So God created man in his [own] image, in the image of God created he him; male and female created he them; And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it." (Gen. 1:27-28)
God created man and woman to be in union for the primary purpose of pro-creation. For Anglican Christians, you are reminded of this at every wedding. "Marriage is not to be entered into...lightly...but...duly considering the causes for which Matrimony was ordained. First, It was ordained for the procreation of children, to be brought up in the fear and nurture of the Lord, and to the praise of his holy Name." (BCP) Of course, sex has other purposes, but this is its primary one. So should couples ever divorce sex and reproduction? It is hard to draw a definitive yes from this passage, but I do believe it is a question that should weigh heavily on the thoughts of Christian people.
Secondly, until recently, homosexuality was universally considered sinful by the Christian Church. Vast majorities of the church still oppose it and rightfully so. The Scriptures teaching from the destruction of Sodom, Gomorrah, and the three cities of the plane to St. Paul's teaching in his letter to the Romans, and the strict forbidding of same sex relations in Leviticus See: Lev. 18:22 and 20:13) is that it is sin for a man to sleep with a man or a woman with a woman.
I wish to pose the question of "why". Now, as Christians, we are obligated to follow the commandments God gives us and don't really have the authority to demand a why. Yet, if we examine the ten commandments, we see a reason behind each one. We are not allowed to murder, because it is the taking of another life created in the image of God. We are not allowed to have gods before Jehovah because he demands our full attention. So what is the "why" behind forbidding homosexuality? I wonder if the reason behind it isn't because it is sex divorced from procreation.
If this is our premise, the sin of Onan recorded in Genesis 37 does not have to be understood as any particular sin, but as falling under the general category of separating sex and sexual pleasure from reproduction.
Update: I posted this blog almost a year ago. Since that time I have become convinced that birth control is a grave moral problem. Sex is intended to be the complete giving of two people two each other, with life being the fruit. Christ gave himself for his bride to give her life. The love of Christ for his Church, and her love for him begets life. Shouldn't then the primary image and reflection of Jesus' love, marriage and sex, also beget life? Isn't contraception then akin to going to Church but never learning about the eternal life offered to us in Christ?