Please do read the summary and appreciate the post as the detailed part is disastrously long.
Summary: There are a few things to keep in mind:
- Adultery (intercourse outside marriage) is a sin.
- Marriage beyond Varna especially pratiloma union (lower Varna man and higher Varna woman) is bad (Refer: 1 and 2
- Killing a Brahmin is one of the major sins (Refer: 5 greatest sins and Manu 8.381) and hence killing as a punishment by the King is out of question for Brahmins)
Hence to prevent pratiloma union a higher punishment by the King was prescribed for lower Varnas (S, V and K) whereas for a Brahmin it was tonsure.
1. Harshness of Punishment
Most Smritis give a harsh punishment, yet some contain prāyaścita too (which are referred to in Manu 11.1176. It is only when one fails to perform the prāyaścita and that a king must intervene (reference needed)
The Samvartta smṛti says:
A Brahmin uniting with a Kshatriya or Vaishya woman is purified by doing the Sāntapana kṛcchra ||153|| Uniting with a Shudra woman (for 15 days or one month) he is purified by consuming cow urine and barley ||154||. By uniting with another family’s Brahmin woman or of ones own family he is purified by Prājāpatya ||157||
If a Shudra united with a Brahmin woman then he is purified by consuming cows urine and barley for one month ||170|| Same for a Kshatriya and Vaishya - 169
So before being harsh, an opportunity is given to do prāyaścita and the harshness by the King comes later. As per Manu (cited below), adultery causes destruction and hence the harsh punishment.
2. Why death penalty only to Shudra
From the below discussion we can see that the general view is death penalty for all (especially pratiloma union ) irrespective of caste, except for the Brahmin there is tonsure rite. So it is not only the shudra who faces such a harsh penalty
The general view expressed is :
Anybody but a Brāhmaṇa shall suffer corporal punishment for adultery.
- Baudhayana DhS Praśna II, Adhyāya 2, Kaṇḍikā 4
The highest fine shall be inflicted for connexion with a woman of equal caste; half that for connexion with a woman of a lower caste; and a man who has connexion with a woman of a higher caste shall be put to death.- Brihaspati 23.10 (I’ve taken this from Manu comparative notes, emphasising Pratiloma union being worse)
Manusmṛti too takes a generalised view initially 8.359 says:
In a case of adultery, a non-Brāhmaṇa deserves the penalty ending in death; as the wives of all the four castes are always the most deserving of protection.
Reason: killing Brahmin is a sin. As per Manu 8.379 (stated below) the death penalty for a Brahmin would be tonsure (shaving off head)
The other generalised prescription of Manu as stated in 8.364-366 is:
If a man of equal status violates an unwilling maiden, he deserves immediate death; but if he violates a willing one, he shall not suffer death.—(364) [in line with the above prescription of death penalty]
If a maiden approaches a superior person, she shall not be made to pay anything; if however she courts an inferior person, she shall be kept confined in the house.—(365) [This is taking about anuloma being less severe than pratiloma]
An inferior man courting a superior maiden deserves death; he who courts a maiden of equal status, shall pay the nuptial fee, if her father so wishes.—(366).
[again showing severity of pratiloma relations]
Yet after the above generalised prescriptions, Manu keeps contradicting itself and goes onto give specific prescriptions for each Varna which (in my opinion maybe interpolated, contemplating the general flow of the above verses and for the simple reason that when a death penalty has been prescribed for all non-Brahmins, why would only the Shudra suffer capital punishment in the succeeding verses?)
However for your satisfaction I will comment on this aspect too, assuming they are valid verses.
Based on the predominance of gunas and owing to the leniency (w.r.t liquor, theft, etc.), a person of a lower Varna was more prone to performing inappropriate acts, including adultery, due to having lesser control over his senses. A Brahmin (Sattva Guna predominant) was commonly expected to control his senses (as mentioned in Bhagavad Gita 18.42) whereas it was not so for a Shudra (Tamo Guna predominant) and he was more likely to be afflicted by Kama.
Manu 8.353 says adulterous intermixing of castes ultimately causes total destruction. As stated above, a child born of a Pratiloma union is considered worse than a child born of an Anuloma union (though the correct and ideal is same Varna union) and therefore a Pratiloma act of adultery results in faster destruction of society.
Accordingly since lower varnas being more prone to committing wrongful acts and especially these acts (pratiloma by a lower Varna) causing greater destruction to society, a higher degree of punishment was prescribed for them. The punishment gradually decreases for each higher Varna. For a shudra it is as you mentioned in the answer. For a Vaishya and Kshatriya we can see a gradual decrease:
The Vaiśya should be fined his entire property after a year’s imprisonment; the Kṣatriya is to be fined one thousand, and be shaved with urine. 8.375
Now there are many contradictions:
Here taking a decreasing view, Manu again goes on to contradict this view making a distinction between protected and unprotected, in the next verse, and prescribe a fine again for the same act:
If the Vaiśya and the Kṣatriya have intercourse with an unprotected Brāhmaṇa woman, the Vaiśya should be committed with five hundred and the Kṣatriya with one thousand. 8.376
But again in the case of a protected Brahmin woman, Manu again contradicts the decreasing punishment view and the fine view to go back to generalised view of death penalty (protected woman) for all except Brahmin:
But both these, when offending against a protected Brāhmaṇa woman, should be punished like a Śūdra, or burnt in a fire of dry grass.—(8.377)
Then again in the next verse he contradicts himself by prescribing fines for Brahmins as stated by you, but once more contradicts himself by saying:
Tonsure has been prescribed as the death-penalty for the Brāhmaṇa; for other castes the penalty would be actual death.—(8.379)
In cases where ‘death’ has been laid down for the Kṣatriya and other castes, it is to be ‘tonsure’ for the Brāhmaṇa. For instance, for adultery, the non-Brāhmaṇa deserves the death-penalty,—the general rule being that ‘the male shall be flayed.’
Probably Manu prescribed fine and death penalty etc but the verses got mixed up.
Vasiṣṭha Dharmasūtra’s view
Vasiṣṭha Dharmasūtra Chapter 21 as cited above prescribes such harsh punishments not only for Shudras but also for adulterous Kshatriyas and Vaishyas, in line with the generalised view of death for adultery (in case of non-Brahmins). The only the difference being in the material which is used to wrap the person before being burnt (again Brahmin killing being a sin is out of question, and this focuses on Pratiloma adultery)
If a Śūdra approaches a female of the Brāhmaṇa caste, (the king) shall cause the Śūdra to be tied up in Vīraṇa grass and shall throw him into a fire
If a Vaiśya approaches a female of the Brāhmaṇa caste, (the king) shall cause the Vaiśya to be tied up in Lohita grass and shall throw him into a fire
If a Kṣatriya approaches a female of the Brāhmaṇa caste, (the king) shall cause the Kṣatriya to be tied up in leaves of Sara grass and shall throw him into a fire
- A Vaiśya who offends) with a female of the Kṣatriya class (shall be treated) in the same manner,
5. And a Śūdra (who offends) with females of the Kṣatriya or Vaiśya castes.
In line with Manu’s prescription of tonsure for Brahmins, in the above cases, the Brahmin lady is supposed to have her head shaved.
I won’t comment on Gautama, because he has dedicate a chapter to punishments for Shudra Varna, none of the others have been mentioned.
Based on the above we can conclude:
- The general view even, as per Manu, is death penalty for a man (of any caste except Brahmin) having a Pratiloma adulterous intercourse and since killing a Brahmin is a sin, tonsure is his death penalty. So it is not only a shudra that faces a harsh punishment
- Though Manu contradicts himself, the basic idea is the severity of punishments for each higher Varna reduces owing to their decreasing likeliness to perform incorrect act. Hence higher punishment for shudra is prescribed.