The men of the higher classes–the brahmanas, ksatriyas and vaisyas–do not beget children in the wombs of lower-class women. Therefore the custom in Vedic society is to examine the horoscopes of a girl and boy being considered for marriage to see whether their combination is suitable. Vedic astrology reveals whether one has been born in the vipra-varna, ksatriya-varna, vaisya-varna or sudra-varna, according to the three qualities of material nature. This must be examined because a marriage between a boy of the vipra-varna and a girl of the sudra-varna is incompatible; married life would be miserable for both husband and wife. Consequently a boy should marry a girl of the same category. Of course, this is trai-gunya, a material calculation according to the Vedas, but if the boy and girl are devotees there need be no such considerations. A devotee is transcendental, and therefore in a marriage between devotees, the boy and girl form a very happy combination.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Sixth Canto, Chapter 02 – Text 26