(27) And God created man in His image, in the image of God He created him; male and female He created them. (28) God blessed them and God said to them, “Be fertile and increase, fill the earth and master it; and rule the fish of the sea, the birds of the sky, and all the living things that creep on earth.”
(1) This is the record of Adam’s line.—When God created man, He made him in the likeness of God; (2) male and female He created them. And when they were created, He blessed them and called them Man.—
(ו) לֹא יִבָּטֵל אָדָם מִפְּרִיָּה וּרְבִיָּה, אֶלָּא אִם כֵּן יֶשׁ לוֹ בָנִים. בֵּית שַׁמַּאי אוֹמְרִים, שְׁנֵי זְכָרִים. וּבֵית הִלֵּל אוֹמְרִים, זָכָר וּנְקֵבָה, שֶׁנֶּאֱמַר, (בראשית ה) זָכָר וּנְקֵבָה בְּרָאָם. נָשָׂא אִשָּׁה, וְשָׁהָה עִמָּהּ עֶשֶׂר שָׁנִים, וְלֹא יָלְדָה, אֵינוֹ רַשַּׁאי לִבָּטֵל. גֵּרְשָׁהּ, מֻתֶּרֶת לִנָּשֵׂא לְאַחֵר. וְרַשַּׁאי הַשֵּׁנִי לִשְׁהוֹת עִמָּהּ עֶשֶׂר שָׁנִים. וְאִם הִפִּילָה, מוֹנֶה מִשָּׁעָה שֶׁהִפִּילָה. הָאִישׁ מְצֻוֶּה עַל פְּרִיָּה וּרְבִיָּה, אֲבָל לֹא הָאִשָּׁה. רַבִּי יוֹחָנָן בֶּן בְּרוֹקָא אוֹמֵר, עַל שְׁנֵיהֶם הוּא אוֹמֵר (בראשית א), וַיְבָרֶךְ אֹתָם אֱלֹהִים וַיֹּאמֶר לָהֶם אֱלֹהִים פְּרוּ וּרְבוּ:
(6) A man shall not abstain from procreation unless he already has children. Beth Shammai says: two males, And Beth Hillel says: male and a female, for it says, “Male and female created he them” (Genesis 5:2). If a man married a woman and lived with her for ten years and she bore no child, he may not abstain [any longer from the duty of propagation]...
(טז) אַף עַל פִּי שֶׁקִּיֵּם אָדָם מִצְוַת פְּרִיָּה וּרְבִיָּה הֲרֵי הוּא מְצֻוֶּה מִדִּבְרֵי סוֹפְרִים שֶׁלֹּא יִבָּטֵל מִלִּפְרוֹת וְלִרְבּוֹת כָּל זְמַן שֶׁיֵּשׁ בּוֹ כֹּחַ. שֶׁכָּל הַמּוֹסִיף נֶפֶשׁ אַחַת בְּיִשְׂרָאֵל כְּאִלּוּ בָּנָה עוֹלָם.
Although a person has fulfilled the Mitzvah of being fruitful and multiplying, he is bound by a Rabbinic commandment not to refrain from being fruitful and multiplying as long as he is physically potent. For anyone who adds a soul to the Jewish people is considered as if he built an entire world.
The Gemara continues to clarify what took place: Granted, Onan engaged in unnatural sexual intercourse with her, as it is written with regard to his act: “That he spilled it on the ground” (Genesis 38:9). However, from where do we derive that Er engaged in unnatural sexual intercourse with her? Rav Naḥman bar Yitzḥak said: As it is written with regard to Onan: “And He slew him also” (Genesis 38:10). This indicates that he, too, died the same death for performing the same transgression as his brother. The Gemara asks: Granted, Onan engaged in anal intercourse because he did not want Tamar to give birth as “he knew that the seed would not be his” (Genesis 38:9). However, with regard to Er, what is the reason he acted in this way? The Gemara responds: He did so in order that she not become pregnant and become less beautiful as a result of her pregnancy.