||Onan, however, realized that the children would not carry his name. Therefore, whenever he came to his brother's wife, he let [the seed] go to waste on the ground, so as not to have children in his brother's name.
Vayeda Onan ki lo lo yihyeh hazara vehayah im-ba el-eshet achiv veshichet artsah levilti netan-zera le-achiv.
||What he did was evil in God's eyes, and He also made him die.
Vayera be'eyney Adonay asher asah vayamet gam-oto.
On the basis of what his father had told him (Sekhel Tov Yov'loth 41:5). Other sources indicate that his mother did not want him to have children by Tamar (Tzava'ath Yehudah 10:6).
|let the seed go to waste...|
It is from here that all the discussions regarding birth control and masturbation are derived (Yevamoth 34b; Niddah 13a; Bereshith Rabbah 85).