The purpose of marrying is to beget a son, because a son is necessary to deliver his father and forefathers from any hellish conditional life in which they may be. Canakya Pandita therefore says, putra-hinam grham sunyam: without a son, married life is simply abominable. King Anga was a very pious king in this life, but because of his previous sinful activity he could not get a son. It is concluded, therefore, that if a person does not get a son it is due to his past sinful life.

From Srila Prabhupada's purport to SB 4.13.31