Why do some Christians oppose IVF when it gives people a chance to have children? I thought Christians were normally "pro life."
That's a really good question, and one that points out some important divisions in our society. Since IVF wasn't invented until very recently, the Bible doesn't mention it in any particular way. However, there are a few aspects in which the Scripture can help us find clarity.
First, in general, the Bible asserts that reproduction normally is within the context of a married husband/wife relationship. Some couples have many children, some have few. And some have none. This is the ordinary way things work. When people look outside of the husband/wife relationship to have children, it regularly brings strife. For further study, look at Abraham's sons Isaac and Ishmael, or Isaac's sons Jacob and Esau. While not all IVF procedures bring a third person into the equation, many do. This can prove to be a relational challenge.
Another, more important issue involved in IVF is the practice of seletion. To use the most accessible terminology, normally many eggs are joined with sperm cells at once, then in one or more steps of selection one or possibly a few are considered viable. Many living beings are created, then most of them are not allowed to develop to maturity.
Most Christians recognize a biblical concept that says human life is inherently valuable no matter the stage of development. Producing a living, growing human, with its own unique biological structure, then purposely ending its growth and life denies its inherent dignity. This is normally at the root of objections not only to IVF, but to all abortion. While life is created, IVF almost always results in the destruction of many lives.
A third observation about reproduction is also in order. Though this is a difficult thing to deal with, the inability to have children is not a sign of God's displeasure or of human failure. Christians can and should recognize that some have many children, some have few, and some have none. Each family, regardless of its size, can trust God in Christ and delight in the life he has provided. Each family has its own way of loving and serving neighbors for the good of all.