Why is intercaste marriage bad?
Intercaste marriages are bad because the offspring produced are genetically inferior. This is especially the case for Pratiloma marriages. Even when the husband is a higher caste than the wife, the wife should ideally be just one caste lower. The best form of marriage is same-caste marriage because it produces children with good qualities.
Marriages where the husband and wife are the same caste, or where the husband is a higher caste than the wife, are called Anuloma (regular order) marriages.
Marriages where the wife is a higher caste than the husband are called Pratiloma (inverse order) marriages.
The four pure castes according to the Manusmriti:
10.4 - The Brāhmaṇa, the Kṣatriya and the Vaiśya are the three twice-born castes; the fourth is the one caste, Śūdra; there is no fifth.
These castes are only generated from a marriage of the same castes:
Manu 10.5 - Among all castes, those only who are born of consorts wedded in the natural order, as virgins of equal status, are to be regarded as the same (as their father).
Āpastamba (2.13.1).—‘Sons begotten by a man who approaches in the proper season a woman of equal caste, who has not belonged to another man, and who has been married legally, have a right to follow the occupations of their castes.’
Even when the husband is a higher caste than the wife, the wife should ideally be just one caste lower:
Mahabharata - Thus as regards the four pure orders, persons beget children invested with their own status upon spouses taken from their own orders as also upon them that are taken from the orders immediately below their own.
And these castes have inherent, natural/genetic predispositions. Ramanujacharya's commentary on Bhagavad Gita verse 18.41 says,
The Sattva-guna becomes dominant in the character of the Brāhmaṇa
through the suppression of the qualities of Rajas and Tamas.
In the Kṣatriya the dominant quality is Rajas through the suppression
of Sattva and Tamas.
The Tamo-guna becomes the inherent nature of the Vaishya, becoming
slightly dominant by suppressing Sattva and Rajas.
While in the Śūdra the Tamo Guna is much more dominant.
Obligations and duties are assigned to them by the Scriptures
according to their inherent dispositions. For the Shastras declare
that the Brāhmaṇas and the others possess particular attributes and
accordingly suggest their duties and occupations.
But children produced from Pratiloma marriages have inferior qualities, lots of Tamoguna, and are wicked in nature:
Yājñavalkya (1.91-95) - The sons born in the inverse order of castes are declared to be bad and those in the regular order, good.
Vaśiṣṭha (18.19).- They quote the following, “One may know by their deeds those who have been begotten secretly, and to whom the stigma of springing from unions in the inverse order of the castes attaches; because they are destitute of virtue and good conduct."
Mahabharata - It is only when they take spouse from castes other than their own, that the children they beget become invested with inferior status. ... The son that is outside the pale of the four orders by uniting with women belonging to the four principal orders, begets offspring that are further degraded in point of status.
Some of their natures:
Mahabharata - The women of the Magadhi caste, by union with these four castes of wicked dispositions produce four others who live by practising deceit. These are Mansa, Swadukara, Kshaudra, and Saugandha. From the Vaideha springs up a cruel and sinful caste that lives by practising deception.
Manu 10.9 - From the Kṣatriya on a Śūdra maiden is born a being called ‘Ugra,’ of the stuff of the Kṣatriya and Śūdra, cruel in his deeds and dealings.
Manu 10.21 - From the ‘Apostate’ Brāhmaṇa is born the evil- natured ‘Bhṛjjakaṇṭaka,’ the Āvantya, the ‘Vāṭadhāna’ the ‘Puṣpadha,’ and the ‘Śaikha.’
Manu 10.29 - Those also beget on each other’s wives several ‘alien’ sons, greatly tainted and despised.
Manu 10.30 - As the Śūdra begets an ‘alien’ being on a Brāhmaṇa woman,—even so an alien propagates on females of the four castes, a still more alien being.
Manu 10.31 - Aliens behaving discordantly, beget fifteen castes, still more alien, disgraced and not disgraced.
Because they have very inferior natures, they are instructed to follow very inferior professions which are appropriate to their natures:
Manu 10.32 - The ‘Dasyu’ begets on the ‘Āyogava,’ the ‘Sairandhra,’ skilled in toilet and attendance, who, though not a slave, makes his living like a slave, and also lives by catching animals.
Manu 10.35 - These three, devoid of caste, are severally born from ‘Āyogava’ women, who wear the clothes of the dead, are ignoble, and eat despised food.
Manu 10.38 - By the ‘Caṇḍāla,’ on the ‘Pukkasa’ woman is begotten the ‘Sopāka,’ whose livelihood consists of death, and who are wicked and despised by good people.
Manu 10.39 - The ‘Niṣāda’ woman bears to the ‘Caṇḍāla’ the son called ‘Antyāvasāyin,’ working in the cremation-ground, despised even by out-casts.
Manu 10.48 - For Niṣādas, the killing of fish; for Āyogavas, carpentry; and for medas, andhras, Cuñcus and Madgus, the killing of wild animals.
Manu 10.49 - For Kṣattṛs, Ugras and Pukkasas, the killing and catching of animals living underground; for Dhigvaṇas, work in leather; and for Veṇas the beating of drums.
Manu 10.50 - Near worshipped trees and cremation-grounds, on hills and in groves, these shall dwell, duly marked, subsisting by their respective occupations.
And the most wicked and evil of all the mixed castes are the Mlecchas, who have given up all Vedic rites, have forgotten their caste, are of mixed-caste, and of unknown caste origin:
Manu 10.43 - But by the omission of the sacred rites, and also by their neglect of Brāhmaṇas, the following Kṣatriya castes have gradually sunk to the position of the low-born.
Manu 10.44 - The Puṇḍrakas, the Coḍas, the Draviḍas, the Kāmbojas, the Yavanas, the Śākas, the Pāradas, the Pahlavas, the Cīnas, the Kirātas, the Daradas and the Khaśas.
Manu 10.45 - All those races of the world which are outside the pale of the people ‘born of the mouth, the arms, the thighs and the feet,’—speaking the ‘barbaric’ or the ‘refined’ language—are called ‘Dasyu.’ [Mleccha]
And the reason for such wicked natures is described below:
Mahabharata - Persons unaccustomed to the performance or for whom sacrifices have not been laid down, and who are deprived of the company and the instructions of the righteous whether numbered among the four principal orders or out of their pale, by uniting themselves with women of other castes, led not by considerations of righteousness but by uncontrolled lust, cause numerous mixed castes to come into existence whose occupations and abodes depend on the circumstances connected with the irregular unions to which they owe their origin.
The nature of the progeny is based on the intention of marriage, the type of marriage, the thought during sexual intercourse, and the caste of the parents:
Baudhāyana (1.11.17).—‘It is well known that as the marriage-forms, so the offspring.’
Āpastamba-Dharmasūtra (2.12.4).—‘To the extent that the form of marriage is the right one, to that same extent is the offspring born thereof of the right kind.’
Manu 3.41 - From the other remaining inferior marriages are born sons, addicted to saying harsh and untrue words, and despisers of the Vedic Dharma.
This is why intercaste marriages are bad, especially Pratiloma intercaste marriages.