What the Bible says about Birth Control

Be fruitful and multiply. (Have as many children as you can.)

Be fruitful, and multiply, and replenish the earth, and subdue it. Genesis 1:28

Be fruitful, and multiply, and replenish the earth. Genesis 9:1

Be ye fruitful, and multiply; bring forth abundantly in the earth, and multiply therein. Genesis 9:7

Every sperm is sacred. Don't waste any.

And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. And the thing which he did displeased the LORD: wherefore he slew him also. Genesis 38:9-10