Genesis 38:9

And Onan knew that the seed should not be his, that a possible first-born son would not perpetuate his name and family, but that of his brother Er; 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. Rather than yield to the custom and be obedient to his father, Onan committed this crime against the divine institution of marriage and its purpose according to the will of God. Such works of the flesh, all too prevalent in our day, when children are no longer desired, are an abomination before the Lord. Where the fear of God still rules, such vices will not be tolerated.