Strong words like this are shocking in our day! Seems like Onan would be pleased to give life to one who would inherit his brother's estate since he was pleased to marry the widow.
Steve Wilkins states, "Human beings were made for the society of marriage. Unless a man was given to gift of celibacy, it is not just a privilege to be married--it is a divine obligation. Men are commanded to seek a wife and should take it as seriously as any other command that God has given...He is a fool who thinks he can live by himself."
A young couple should be prepared to have children when they get married. You can grow in your relationship during marriage and child bearing. It's never perfect. To divorce the marriage relationship from bearing children is perverse. Seems like those who do often end up setting aside the marriage relationship for just living in the same house but in separate bedrooms. Everything should be done in faith to the Glory of God.
Some who are the most demanding that their wife use birth control and get furious with her for getting pregnant are the most proud of those "unwanted" children and later their grandchildren.