God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. - Genesis 1:28

Right there.

But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother. What he did was wicked in the Lord’s sight; so the Lord put him to death also. - Genesis 38:9-10

Onan is having sex, not for babies, but for pleasure, and God sees it as wicked and strikes him dead. And before you give the common argument that God is punishing Onan for not fulfilling his duty to bear kids for his brother, here is the actual punishment for that:

If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband’s brother shall take her and marry her and fulfill the duty of a brother-in-law to her. The first son she bears shall carry on the name of the dead brother so that his name will not be blotted out from Israel. However, if a man does not want to marry his brother’s wife, she shall go to the elders at the town gate and say, “My husband’s brother refuses to carry on his brother’s name in Israel. He will not fulfill the duty of a brother-in-law to me.” Then the elders of his town shall summon him and talk to him. If he persists in saying, “I do not want to marry her,” his brother’s widow shall go up to him in the presence of the elders, take off one of his sandals, spit in his face and say, “This is what is done to the man who will not build up his brother’s family line.” That man’s line shall be known in Israel as The Family of the Unsandaled. - Deuteronomy 25:5-10

Notice that the punishment is not death but humiliation.

Here is Martin Luther:

Onan must have been a malicious and incorrigible scoundrel. This is a most disgraceful sin. It is far more atrocious than incest and adultery. We call it unchastity, yes, a Sodomitic sin. For Onan goes in to her; that is, he lies with her and copulates, and when it comes to the point of insemination, spills the semen, lest the woman conceive. Surely at such a time the order of nature established by God in procreation should be followed (Lectures on Genesis: Chapters 38-44; 1544; in Luther's Works, Vol. 7, 20-21)

The rest of the populace is more wicked than even the heathen themselves. For most married people do not desire offspring. Indeed, they turn away from it and consider it better to live without children, because they are poor and do not have the means with which to support a household. . . . But the purpose of marriage is not to have pleasure and to be idle but to procreate and bring up children, to support a household. . . . Those who have no love for children are swine, stocks, and logs unworthy of being called men and women; . . . (Lectures on Genesis: Chapters 26-30; Luther's Works, vol. 5, 325-328; vol. 28, 279; commentary on the birth of Joseph; cf. Luther's Works, vol. 45, 39-40)

You will find many to whom a large number of children is unwelcome, as though marriage had been instituted only for bestial pleasures and not also for the very valuable work by which we serve God and men when we train and educate the children whom God has given us. (In Ewald M. Plass, What Luther Says, an Anthology, two volumes, St. Louis: Concordia Publishing House, 1959, Vol. II, #2834)