As in title. Is it by birth? Especially righteous ones like Prahalada, Bali and to some extend Ravana. And many of them are sons of saints. So what defines an Asura?
Lets understand the difference between Asura and Rakshasa.
Asura : is not with birth, anyone who is not righteous can be termed as asura, its more related to the deeds of any living entity, even animals.
Rakshasa: is a kind of yoni altogether, it is a species who is like human, has brains but is carnivorous, they can be good or bad, like good one being Prahalad and Bali, they have same nature and motive as humans, to find the Supreme Being, the only difference is there eating habits which make them more dangerous and Tamsik than humans.
Generally asura means someone who is opposite or anti of the suras. Sura means the devas and are called so because they took Varuni (goddess of wine or sura) when she emerged from the churning of the ocean.
So Asuras are danavas or demons and they are such by their birth as they were born of Danu, a wife of rishi Kashyapa. The Mahabharata narrates about them and lists names of some well known asuras including Prahlada. Prahlada, as an asura, although should have been envious to Vishnu, was however very devoted to Him for which his father wanted to kill Prahlada.
In the Puranas like Shrimad Bhagavatam there is also mention of Brahma creating the gods and the asuras. And one nature of them is given that they are very fond of copulation:
Lord Brahmā then gave birth to the demons from his buttocks, and they were very fond of sex. Because they were too lustful, they approached him for copulation. [SB - 3.20.23]