Another aspect was that had Onan sired a child by his brother's widow, that child would have been counted as his brother's son. By not doing so, he in effect left his brother childless, with no one to carry on his lineage.
Continuing a man's line was considered extremely important in that time. In Jeremiah 22, God curses Jecohiah (Coniah) by decreeing that they "write this man childless." By doing this, his children were considered illegitimate, and unable to inherit his lineage.
24 As I live, saith the LORD, though Coniah the son of Jehoiakim king of Judah were the signet upon my right hand, yet would I pluck thee thence;
25 And I will give thee into the hand of them that seek thy life, and into the hand of them whose face thou fearest, even into the hand of Nebuchadrezzar king of Babylon, and into the hand of the Chaldeans.
26 And I will cast thee out, and thy mother that bare thee, into another country, where ye were not born; and there shall ye die.
27 But to the land whereunto they desire to return, thither shall they not return.
28 Is this man Coniah a despised broken idol? is he a vessel wherein is no pleasure? wherefore are they cast out, he and his seed, and are cast into a land which they know not?
29 O earth, earth, earth, hear the word of the LORD.
30 Thus saith the LORD, Write ye this man childless, a man that shall not prosper in his days: for no man of his seed shall prosper, sitting upon the throne of David, and ruling any more in Judah.