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.”

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).

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.)

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.