מִצְוַת פְּרִיָּה וּרְבִיָּה – בְּרֵאשִׁית יֵשׁ בָּהּ מִצְוַת עֲשֵׂה אַחַת, וְהִיא מִצְוַת פְּרִיָּה וּרְבִיָּה, שֶׁנֶּאֱמַר: (בראשית א כח) וַיְבָרֶךְ אֹתָם אֱלֹהִים וַיֹּאמֶר לָהֶם אֱלֹהִים פְּרוּ וּרְבוּ. The commandment of procreation: [Parashat] Bereshit has one positive commandment and that is the commandment of procreation, as it is stated (Genesis 1:28), “And God blessed them and told them to procreate.”
מִשָּׁרְשֵׁי מִצְוָה זוֹ, כְּדֵי שֶׁיִּהְיֶה הָעוֹלָם מְיֻשָּׁב (גיטין מא, ב במשנה), שֶׁהַשֵּׁם בָּרוּךְ הוּא חָפֵץ בְּיִשּׁוּבוֹ, כְּדִכְתִיב: (ישעיהו מה יח) לֹא תֹהוּ בְרָאָהּ לָשֶׁבֶת יְצָרָהּ. וְהִיא מִצְוָה גְּדוֹלָה שֶׁבְּסִבָּתָהּ מִתְקַיְּמוֹת כָּל הַמִּצְוֹת בָּעוֹלָם, כִּי לִבְנֵי אָדָם נִתְּנוּ וְלֹא לְמַלְאֲכֵי הַשָּׁרֵת (ברכות כה, ב). It is from the roots of this commandment (i.e. the reason behind this commandment) that the world should be settled (Gittin 41b) because God wants the world to be settled, as it says (Isaiah 45:18), “I did not create it for naught, but [rather] formed it for habitation.” This is a great commandment, through which all the commandments are observed, as [the Torah] was given to people and not to the ministering angels (Berakhot 25b).
דִּינֵי הַמִּצְוָה, מָתַי חַיָּב אָדָם לַעֲסֹק בָּהּ, וְכַמָּה בָּנִים יִהְיֶה לוֹ וְיִפָּטֵר, וּמֵאֵיזוֹ מִצְוֹת הוּא פָּטוּר בְּעִסְקוֹ בָּזוֹ, וְיֶתֶר פְּרָטֶיהָ מְבֹאָרִים בִּיבָמוֹת בְּפֶרֶק ו' (דף סא, ב וסב, ב) וּבִבְרָכוֹת (דף טז, א). (שו"ע אבן העזר סימן א ואו"ח סימן ע סעיף ג). The laws of this commandment — when a man is obligated to be involved with it; how many sons he needs to have; what other commandments he is exempt from due to his involvement in procreation; and the rest of its details — are [all] explained in the sixth chapter of Yevamot and in Berachot. (See Shulchan Arukh, Even HaEzer 1 and Orach Chaim 70:3.)
וְנוֹהֶגֶת בְּכָל מָקוֹם וּבְכָל זְמַן (קדושין לו, ב) וְחַיָּב אָדָם לְהִשְׁתַּדֵּל בָּהּ מִשֶּׁהוּא רָאוּי לָהּ, וְהוּא הַזְּמַן שֶׁנָּתְנוּ חֲכָמִים ז"ל (אבות פ"ה מכ"א) לִשָּׂא אִשָּׁה. וּמִצְוָה זוֹ אֵינָהּ מֻטֶּלֶת עַל הַנָּשִׁים, וְהַמְּבַטְּלָה בִּטֵּל עֲשֵׂה וְעָנְשׁוֹ גָּדוֹל מְאֹד, (קדושין כט, ב) שֶׁמַּרְאֶה בְּעַצְמוֹ, שֶׁאֵינוֹ רוֹצֶה לְהַשְׁלִים חֵפֶץ הַשֵּׁם לְיַשֵּׁב עוֹלָמוֹ. And [it] is practiced in every place and at all times (Kiddushin 36b), and a person is obligated to be involved with it from when it is fitting for him, and this is the age which the Sages (Avot 5:21) designated to marry a woman. And this commandment is not incumbent upon women. And one who negates it violates a positive commandment; and his punishment is very great (Kiddushin 29b), as he shows that he does not want to fulfill God’s will to settle His world.