SB 4.8.1

maitreya uvāca
sanakādyā nāradaś ca
 ṛbhur haṁso ’ruṇir yatiḥ
naite gṛhān brahma-sutā
 hy āvasann ūrdhva-retasaḥ
Synonyms: 
maitreyaḥ uvāca — Maitreya said; sanaka-ādyāḥ — those headed by Sanaka; nāradaḥ — Nārada; ca — and; ṛbhuḥ — Ṛbhu; haṁsaḥ — Haṁsa; aruṇiḥ — Aruṇi; yatiḥ — Yati; na — not; ete — all these; gṛhān — at home; brahma-sutāḥ — sons of Brahmā; hi — certainly; āvasan — did live; ūrdhva-retasaḥ — unadulterated celibates.
Translation: 
The great sage Maitreya said: The four great Kumāra sages headed by Sanaka, as well as Nārada, Ṛbhu, Haṁsa, Aruṇi and Yati, all sons of Brahmā, did not live at home, but became ūrdhva-retā, or naiṣṭhika-brahmacārīs, unadulterated celibates.
Purport: 

The system of brahmacarya has been current since the birth of Brahmā. A section of the population, especially male, did not marry at all. Instead of allowing their semen to be driven downwards, they used to lift the semen up to the brain. They are called ūrdhva-retasaḥ, those who lift up. Semen is so important that if, by the yogic process, one can lift the semen up to the brain, he can perform wonderful work — one’s memory is enabled to act very swiftly, and the duration of life is increased. Yogīs can thus perform all kinds of austerity with steadiness and be elevated to the highest perfectional stage, even to the spiritual world. Vivid examples of brahmacārīs who accepted this principle of life are the four sages Sanaka, Sanandana, Sanātana and Sanat-kumāra, as well as Nārada and others.

Another significant phrase here is naite gṛhān hy āvasan, “they did not live at home.” Gṛha means “home” as well as “wife.” In fact, “home” means wife; “home” does not mean a room or a house. One who lives with a wife lives at home; otherwise, a sannyāsī or brahmacārī, even though he may live in a room or in a house, does not live at home. That they did not live at home means that they did not accept a wife, and so there was no question of their discharging semen. Semen is meant to be discharged when one has a home, a wife and the intention to beget children; otherwise there is no injunction for discharging semen. These principles were followed from the beginning of creation, and such brahmacārīs never created progeny. This narration has dealt with the descendants of Lord Brahmā from Manu’s daughter Prasūti. Prasūti’s daughter was Dākṣāyaṇī, or Satī, in relation to whom the story of the Dakṣa yajña was narrated. Maitreya is now explaining about the progeny of the sons of Brahmā. Out of the many sons of Brahmā, the brahmacārī sons headed by Sanaka and Nārada did not marry at all, and therefore there is no question of narrating the history of their descendants.