Maharaja Anga was very pure because of the purification of semen in the family of Maharaja Dhruva. However, his semen became contaminated in association with his wife, Sunitha, who happened to be the daughter of death personified
SB Canto 4
The sages decided that the descendants of the family of the saintly King Aṅga should not be stopped, for in this family the semen was very powerful and the children were prone to become devotees of the Lord.
The purity of hereditary succession is called amogha-vīrya. The pious seminal succession in the twice-born families of the brāhmaṇas and kṣatriyas especially, as well as in the families of vaiśyas also, must be kept very pure by the observation of the purificatory processes beginning with garbhādhāna-saṁskāra, which is observed before conceiving a child. Unless this purificatory process is strictly observed, especially by the brāhmaṇas, the family descendants become impure, and gradually sinful activities become visible in the family. Mahārāja Aṅga was very pure because of the purification of semen in the family of Mahārāja Dhruva. However, his semen became contaminated in association with his wife, Sunīthā, who happened to be the daughter of death personified. Because of this polluted semen, King Vena was produced. This was a catastrophe in the family of Dhruva Mahārāja. All the saintly persons and sages considered this point, and they decided to take action in this matter, as described in the following verses.