In Kṛṣṇamiśra’s (eleventh century) allegorical
play, Rise of Moon-like Intellect (Prabodhacandrodaya),
Mahāmoha (Great Delusion), an avowed materialist, declares:   

tulyatve vapuṣāṃ mukhādyavayavair
varṇakramaḥ kīdṛśo…| 2.18ab
If the bodies are alike in their different parts,
the mouth, etc., how can there be a hierarchy of castes? (Trans. by S.K. Nambiar)

śuddhir
vaṃśa dvayī śuddhau pitroryadekaśaḥ | 
tadanantakulādopādadoṣā
jātirasti kā || 17.40

īrṣyayā
rakṣato nārīrdhikulasthitidāmbhikān | 
smarāndhatvāviśeṣe’pi
tathā naramarakṣataḥ || 17.42

tṛṇānīva
ghṛṇāvādān vidhūnaya vadhūranu | 
tavāpi
tādṛśasyaiva kā ciraṃ janavañcanā ||  17.58 

Since purity of caste is possible only in the case of purity
on each side of both families of the grandparents, what caste is pure by the
purity of limitless generations?

Fie on those who boast of family dignity! They hold women in
check out of jealousy; but do not likewise restrain men, though the blindness
of passion is common to both!

Spurn all censorious statements about women as not worth a
straw. Why dost thou constantly cheat people when thou, too, art as bad as
women?(Trans. by K.K. Handiqui) 

Both the authors intended
to depict the Cārvākas as heretics and non-believers. Defiance of the caste
system was considered a heretical idea and hence deserved censure.

Is there any truth in
labelling the Cārvākas as opposed to the caste system? I think there is. Two
oft-quoted genuine aphorisms attributed to the Cārvākas say that the human body
is a combination of four natural elements, namely, earth, air, fire and water
(I.2-3).[1]
Apparently the Cārvākas gave no credence to the late Vedic idea that the Brahmaṇas,
Rājanyas (warriors), Vaiśyas (agriculturists and traders), and Śūdras (manual
workers) were different parts of the supreme person called puruṣa (Ṛgveda
10.90.11-12): 

yat
puruṣaṃ viadadhuḥ, katidhā vi akalpayan? 
mukhaṃ
kim asya? kau bāhu? kā ῡrῡ pādā ucyete?

brāhmaṇo
‘sya mukham āsīd, bāhu rājaniaḥ kṛtaḥ; 
ῡrῡ
tad asya yad vaiśyaḥ padbhyāṃ śῡdro ajāyata. 

When they divided Puruṣa,
into how many parts did they dispose him? What (did) his mouth (become)? What
are his two arms, his too thighs, his two feet called? 

His mouth was the Brāhaman
[Brāhamaṇa], his two arms were made the warrior, his two thighs the Vaiśya;
from his two feet the Śῡdra was born. (trans.  A.A. Macdonell,
pp.200-01) 

This was a convenient way
of explaining why acceptance and observance of the hierarchy of castes was
obligatory. The law books insist on the preservation and continuation of the
caste system. The Cārvākas cared nothing for these sacred texts. Hence it is
quite probable that the Cārvākas had no faith in the so-called divine origin of
castes and did not observe caste rules in social life. A verse attributed to
the Cārvākas runs as follows:  

na
svarga nāpavargo vā naivātmā pāralaukikaḥ | 
naiva varṇāśramādīnāṃ
kriyāśca phaladāyikāḥ ||  

There is no heaven, no
final liberation, nor any soul inthe other-world. 
Nor
do the actions of the four castes, orders, etc. produce any real effect.[2] 

The Carvakas against Gender Discrimination

As to the defence of women and treating them as equal to men,
the Cārvākas apparently were very much anti-sexist. They did not believe, as
Manu did, that women in general were basically untrustworthy, were not entitled
to study the Vedas, and were never to earn freedom but should always be under
their fathers’, husbands’ and sons’ protection and surveillance (Manusmṛti, 9.10-20). The Cārvākas’ claim
of the equality of the sexes quite logically follows from their basic
anti-śāstric stance. Being freethinkers, they could also very well be free from
all prejudices against women that are rampant in the law books of ancient
India. They did not admit word (śabda),
that is, verbal testimony, as a valid instrument of cognition (pramāṇa). So they were not under any
compulsion to accept what the Brahmaṇcal law books declared as something sacrosanct.
This is why Sāyaṇa-Mādhava could make them say: ”[T]here is no more reason for
believing on another’s word that smoke and fire are connected, than for our
receiving the ipse dixit of Manu,
&c.” dhῡmadhῡma-dhvajayor avinābhāvo ’stīti
vacanamātre manvādivad viśvāsābhāvāc ca. [3] 

To sum up then: materialism
in India, particularly the Cārvāka/Lokāyata,  appeared heretical to the powers that be, not
only in respect of its ontology but also because of its social outlook. This is
why two Vedantin poets considered it necessary to present it in poor light, as
an enemy of the established order which is founded on the varṇa system and patriarchy. 

One word more. Eli Franco once suggested perceptively: “[A]ll the
Lokayātikas were fighting for… was ultimately to found social and political
institutions independently of religious dogma…”.[4] He might
have had in his mind Frauwallner’s view that materialism in India was created
for the Realpolitikers. I do not think so, as I have shown elsewhere.[5] I would, however,
heartily agree with Franco’s suggestion. The Cārvākas did have a vision of an
ideal society in which organised religion would have no room, and there would
be no varṇa and gender discriminations. Their
approach was thoroughly rational and they denounced such discriminations as
impediments to founding a society based on equality of rights and
opportunities. In this sense the social outlook of the Cārvākas was essentially
democratic.

Ajita Kesakambal’s ‘worldview’

O King, there is no (consequence to) alms-giving, sacrifice
or oblation. A good or bad action produces no result. This world does not
exist, nor does the other world.  There
is no mother, no father. There is no rebirth of beings after death. In this
world, there are no samanas [Śramaṇas] or brāhmaṇas established in the
Noble Path and accomplished in good practice, who, through direct knowledge (i.e., magga insight) acquired by their own efforts, can expound on this
world and the other world. This being is but a compound of the four great
primary elements; after death, the earth-element (or element of extension)
returns and goes back to the body of the earth, the water-element (or element
of cohesion) returns and goes back to the body of water, the fire-element (or
element of thermal energy) returns and goes back to the body of fire, and the
air-element (or element of motion) returns and goes back to the body of air,
while the mental faculties pass on into space. The four pall-bearers and the
bier (constituting the fifth) carry the corpse. The remains of the dead can be
seen up to the cemetery where bare bones lie graying like the colour of the
pigeons. All alms-giving ends in ashes. Fools prescribe alms-giving; and some
assert that there is such a thing as merit in alms-giving; but their words are
empty, false and nonsensical. Both the fool and the wise are annihilated and
destroyed after death and dissolution of their bodies. Nothing exists after
death.’ (Ten Suttas, p.83,
translation slightly modified.)