SB 4.21.46
putrena jayate lokan
iti satyavati srutih
brahma-danda-hatah papo
yad veno ’tyatarat tamah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
They all declared that the Vedic conclusion that one can conquer the heavenly planets by the action of a putra, or son, was fulfilled, for the most sinful Vena, who had been killed by the curse of the brahmanas, was now delivered from the darkest region of hellish life by his son, Maharaja Prthu.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
According to the Vedic version, there is a hellish planet called Put, and one who delivers a person from there is called putra. The purpose of marriage, therefore, is to have a putra, or son who is able to deliver his father, even if the father falls down to the hellish condition of Put. Maharaja Prthu’s father, Vena, was a most sinful person and was therefore cursed to death by the brahmanas. Now all the great saintly persons, sages and brahmanas present in the meeting, after hearing from Maharaja Prthu about his great mission in life, became convinced that the statement of the Vedas had been fully proved. The purpose of accepting a wife in religious marriage, as sanctioned in the Vedas, is to have a putra, a son qualified to deliver his father from the darkest region of hellish life. Marriage is not intended for sense gratification but for getting a son fully qualified to deliver his father. But if a son is raised to become an unqualified demon, how can he deliver his father from hellish life? It is therefore the duty of a father to become a Vaisnava and raise his children to become Vaisnavas; then even if by chance the father falls into a hellish life in his next birth, such a son can deliver him, as Maharaja Prthu delivered his father.