SB 3.13.11

sa tvam asyām apatyāni
 sadṛśāny ātmano guṇaiḥ
utpādya śāsa dharmeṇa
 gāṁ yajñaiḥ puruṣaṁ yaja
Word for word: 
saḥ — therefore that obedient son; tvam — as you are; asyām — in her; apatyāni — children; sadṛśāni — equally qualified; ātmanaḥ — of yourself; guṇaiḥ — with the characteristics; utpādya — having begotten; śāsa — rule; dharmeṇa — on the principles of devotional service; gām — the world; yajñaiḥ — by sacrifices; puruṣam — the Supreme Personality of Godhead; yaja — worship.
Translation: 
Since you are my very obedient son, I ask you to beget children qualified like yourself in the womb of your wife. Rule the world in pursuance of the principles of devotional service unto the Supreme Personality of Godhead, and thus worship the Lord by performances of yajña.
Purport: 

The purpose of the material creation by Brahmā is clearly described herein. Every human being should beget nice children in the womb of his wife, as a sacrifice for the purpose of worshiping the Supreme Personality of Godhead in devotional service. In the Viṣṇu Purāṇa (3.8.9) it is stated:

varṇāśramācāravatā
 puruṣeṇa paraḥ pumān
viṣṇur ārādhyate panthā
 nānyat tat-toṣa-kāraṇam

“One can worship the Supreme Personality of Godhead, Viṣṇu, by proper discharge of the principles of varṇa and āśrama. There is no alternative to pacifying the Lord by execution of the principles of the varṇāśrama system.”

Viṣṇu worship is the ultimate aim of human life. Those who take the license of married life for sense enjoyment must also take the responsibility to satisfy the Supreme Personality of Godhead, Viṣṇu, and the first stepping-stone is the varṇāśrama-dharma system. Varṇāśrama-dharma is the systematic institution for advancing in worship of Viṣṇu. However, if one directly engages in the process of devotional service to the Supreme Personality of Godhead, it may not be necessary to undergo the disciplinary system of varṇāśrama-dharma. The other sons of Brahmā, the Kumāras, directly engaged in devotional service, and thus they had no need to execute the principles of varṇāśrama-dharma.