Manava Dharmashastra 3.42

3.42

samāsato vivāhānāṃ phalapradarśanametat | ye yasya vivāhā vihitāste'ninditāstairuḍhānāṃ yā prajā putrādilakṣaṇā sā'nindyā bhavati praśasyetyarthaḥ | ninditaiḥ pratiṣiddhaiḥ ninditā garhitā | tasmādduḥkhabhāginī prajā mā bhūditi nindyānvivarjayet |

Medhātithi’s commentary (manubhāṣya):
This verse sums up the results of marriages briefly. Those marriages are called ‘unblamable’ which have been sanctioned by the scriptures: and the offspring—in the shape of son, etc.—born from wives wedded by those forms of marriage is ‘unblamable,’—i.e., praiseworthy.

‘From blameworthy’—i.e., prohibited—‘marriages’ is born ‘blameworthy,’ defective, child.

‘Therefore’—with the view that such children may not be born as become a source of pain, ‘one should avoid the blamable marriages’—(42)