Gen 38:8-10

8 Then Judah said to Onan, "Go in to your brother's wife, and perform your duty as a brother-in-law to her, and raise up offspring for your brother."

9 And Onan knew that the offspring would not be his; so it came about that when he went in to his brother's wife, he wasted his seed on the ground, in order not to give offspring to his brother.

10 But what he did was displeasing in the sight of the LORD; so He took his life also.
NASB

This text doesn't have anything to do with whether or not birth control is right or wrong as we read in the commentary below but that Onan didn't fulfill his Old Testament responsibilty.

Gen 38:6-11

38:6-11. Judah's first son Er died because he was wicked. By the custom of the levirate (from Latin levir, "husband's brother") law of marriage, the second son, Onan, was to marry Tamar, the widow of his brother, and raise up offspring for his brother. However, Onan repeatedly used that law for sexual gratification. He took advantage of the situation, but refused the responsibility that went with it. So God took his life too.

(from Bible Knowledge Commentary/Old Testament Copyright © 1983, 2000 Cook Communications Ministries; Bible Knowledge Commentary/New Testament Copyright © 1983, 2000 Cook Communications Ministries. All rights reserved.)

Children are a blessing form the Lord,  but there is no Biblical condemnation for birth control.The argument that God should be in control of how many children we have does not take into account that God does not only lead through circumstance, but He can also speak to us and direct us to wisely follow His will.