The Law Code of Yājñavalkya 1.3.37

1.3.37

ratiputradharmārthatvena vivāhas trividhaḥ | tatra putrārtho dvividhaḥ nityaḥ kāmyaś ca | tatra nitye prajārthe savarṇaḥ śrotriyo varaḥ ity anena savarṇā mukhyā darśitā | idanīṃ kāmye nityasaṃyoge cānukalpo vaktavya ity ata āha |

Marriage is threefold, having pleasure, offspring, and duty as its ends. Among these, the one for offspring is twofold: obligatory and optional. There, in the obligatory one for offspring, by the words "of the same social class, a Vedic scholar, the groom," a woman of the same class was shown to be primary. Now, since a secondary substitute must be stated for the optional [marriage] and for the obligatory conjunction, he says: