Kommentar zu Bereschit 38:10

גם אותו. According to our sages Er also died for this same type of sin, as he deliberately wasted his semen by ejaculating prematurely to prevent his wife from becoming pregnant. He wanted to preserve her physical beauty.

גם אותו, both brothers had been guilty of the same sin.

וימת גם אותו, “He killed him also.” The word גם is proof that Er and Onan died for the same sin. Our sages in Yevamot 34 say that it is easy to understand why Onan should have died for this sin seeing he had violated his father’s instructions to marry Tamar and to have children by her (verse 8). He thought that these children would not be accounted as his own but as his deceased brother’s Er, hence he deliberately wasted his semen. The question is why did Er act in such a fashion? Why did he not want children? The Talmud answers that he did not want Tamar to become disfigured through a pregnancy. He wanted to preserve her beauty, i.e. to treat her as a sex object rather than as the mother of his children. This explains why Yehudah had stressed to Onan (verse 8) “and maintain seed for your brother.” Seeing that Yehudah did not add the word המת, “the deceased,” when instructing Onan to have children with Tamar, Onan realised that when Tamar would have children these would not even be considered as his children but as his late brother’s. In other words, he would not even be compensated for Tamar losing her youthful beauty.
The meaning of the letter ל in the word לאחיך in verse 8 may be understood as similar to אמרי לי אחי הוא, “say concerning me he is my brother” (Genesis 20,13). The verse teaches us the lesson that if someone deliberately wastes his semen, i.e. שחת ארצה, this was the sin of the generation which perished during the deluge. We find the word כי השחית כל בשר על הארץ in connection with that generation, i.e. the same word as we find describing Onan’s sin (Nidah 12). The reason the Torah calls the firstborn of Yehudah ער was because these letters are the same as רע, “evil” in G’d’s eyes [same letters in a different sequence. Ed.] Seeing the Torah mentions the word רע in connection with both Er, Onan, and the people of Sodom (Genesis 13,13), it stands to reason that all of these people were guilty of the sin of letting their semen go to waste deliberately.