SB 9.18.5
sri-rajovaca
brahmarsir bhagavan kavyah
ksatra-bandhus ca nahusah
rajanya-viprayoh kasmad
vivahah pratilomakah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Pariksit said: Sukracarya was a very powerful brahmana, and Maharaja Yayati was a ksatriya. Therefore I am curious to know how there occurred this pratiloma marriage between a ksatriya and a brahmana.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
According to the Vedic system, marriages between ksatriyas and ksatriyas or between brahmanas and brahmanas are the general custom. If marriages sometimes take place between different classes, these marriages are of two types, namely anuloma and pratiloma. Anuloma, marriage between a brahmana and the daughter of a ksatriya, is permissible, but pratiloma, marriage between a ksatriya and the daughter of a brahmana, is not generally allowed. Therefore Maharaja Pariksit was curious about how Sukracarya, a powerful brahmana, could accept the principle of pratiloma. Maharaja Pariksit was eager to know the cause for this uncommon marriage.