ואתם פרו ורבו. לְפִי פְּשׁוּטוֹ הָרִאשׁוֹנָה לִבְרָכָה וְכָאן לְצִוּוּי. וּלְפִי מִדְרָשׁוֹ לְהַקִּישׁ מִי שֶׁאֵינוֹ עוֹסֵק בִּפְרִיָּה וּרְבִיָּה לְשׁוֹפֵךְ דָּמִים (יבמות ס"ג):
ואתם פרו ורבו AND YE, BE FRUITFUL AND MULTIPLY — According to the real sense, the first time this mas said to man (Genesis 9:1) it was said as a blessing (an assurance that they would be fruitful); here it is a command (Ketubot 5a). According to the Midrashic explanation this command is mentioned here after the mention of murder in order to liken one who abstains from having children to one who sheds blood (Yevamot 63b).