A man or woman who indulges in sexual intercourse with an unworthy member of the opposite sex is punished after death by the assistants of Yamarāja in the hell known as Taptasūrmi. There such men and women are beaten with whips. The man is forced to embrace a red-hot iron form of a woman, and the woman is forced to embrace a similar form of a man. Such is the punishment for illicit sex.
(Srimad Bhagavatam----5:26:20----translation).

A person who has sexual relations with a woman who is not to be enjoyed is put into the hell known as Taptasūrmi.
(Srimad Bhagavatam----5:26--summary).

Intelligent persons who can see properly may look into the general conditions of the living entities who are wandering in the cycle of the 8,400,000 spieces of life, as well as in different classes of human beings. It is said that there is an everlasting belt of water called the River Vaitaraṇī at the entrance of the plutonic planet of Yamarāja, who punishes sinners in different manners. After being subjected to such sufferings, a sinner is awarded a particular species of life according to his deeds in the past. Such living entities as are punished by Yamarāja are seen in different varieties of conditioned life. Some of them are in heaven, and some of them are in hell. Some of them are brāhmaṇas, and some of them are misers.
(Srimad Bhagavatam----2:2:7----purport).

Material existence is compared herein to the great ocean of nescience. Another name of this ocean is Vaitaraṇī. In that Vaitaraṇī Ocean, which is the Causal Ocean, there are innumerable universes floating like footballs. On the other side of the ocean is the spiritual world of Vaikuṇṭha, which is described in Bhagavad-gītā (8.20) as paras tasmāt tu bhāvo 'nyaḥ. Thus there is an ever-existing spiritual nature which is beyond this material nature. Even though all the material universes are annihilated again and again in the Causal Ocean, the Vaikuṇṭha planets, which are spiritual, exist eternally and are not subject to dissolution. The human form of life gives the living entity a chance to cross the ocean of nescience, which is this material universe, and enter into the spiritual sky. Although there are many methods or boats by which one can cross the ocean, the Kumāras recommend that the King take shelter of the lotus feet of the Lord, just as one would take shelter of a good boat. Nondevotees, who do not take shelter of the Lord's lotus feet, try to cross the ocean of nescience by other methods (karma, jñāna and yoga), but they have a great deal of trouble. Indeed, sometimes they become so busy simply enjoying their troubles that they never cross the ocean. There is no guarantee that the nondevotees will cross the ocean, but even though they manage to cross, they have to undergo severe austerities and penances. On the other hand, anyone who takes to the process of devotional service and has faith that the lotus feet of the Lord are safe boats to cross that ocean is certain to cross very easily and comfortably.
(Srimad Bhagavatam----4:22:40----purport).

A person who is born into a responsible family—such as a kṣatriya, a member of royalty or a government servant—but who neglects to execute his prescribed duties according to religious principles, and who thus becomes degraded, falls down at the time of death into the river of hell known asVaitaraṇī. This river, which is a moat surrounding hell, is full of ferocious aquatic animals. When a sinful man is thrown into the River Vaitaraṇī, the aquatic animals there immediately begin to eat him, but because of his extremely sinful life, he does not leave his body. He constantly remembers his sinful activities and suffers terribly in that river, which is full of stool, urine, pus, blood, hair, nails, bones, marrow, flesh and fat.
(Srimad Bhagavatam----5:26:22----translation).

There is a river known as Vaitaraṇī between the material and spiritual worlds, and one must cross this river to reach the other side, or the spiritual world. This is an extremely difficult task. As the Lord says in Bhagavad-gītā (7.14), daivī hy eṣā guṇa-mayī mama māyā duratyayā: "This divine energy of Mine, consisting of the three modes of material nature, is difficult to overcome." The same word duratyaya, meaning "very difficult," is used here. Therefore one cannot surpass the stringent laws of material nature except by the mercy of the Supreme Lord. Nonetheless, although all materialists are baffled in their plans, they try again and again to become happy in this material world. Therefore they have been described as vimūḍha—first-class fools. As for Prahlāda Mahārāja, he was not at all unhappy, for although he was in the material world, he was full of Kṛṣṇa consciousness. Those who are Kṛṣṇa conscious, trying to serve the Lord, are not unhappy, whereas one who has no assets in Kṛṣṇa consciousness and is struggling for existence is not only foolish but extremely unhappy also. Prahlāda Mahārāja was happy and unhappy simultaneously. He felt happiness and transcendental bliss because of his being Kṛṣṇa conscious, yet he felt great unhappiness for the fools and rascals who make elaborate plans to be happy in this material world.
(Srimad Bhagavatam----7:9:43----purport).

A person who enjoys sexual relations with animals is put into the hell known as Vajrakaṇṭaka-śālmalī.
(Srimad Bhagavatam----5:26--summary).

A person who indulges in sex indiscriminately—even with animals—is taken after death to the hell known as Vajrakaṇṭaka-śālmalī. In this hell there is a silk-cotton tree full of thorns as strong as thunderbolts. The agents of Yamarāja hang the sinful man on that tree and pull him down forcibly so that the thorns very severely tear his body.
(Srimad Bhagavatam----5:26:21----translation).

The sexual urge is so strong that sometimes a man indulges in sexual relations with a cow, or a woman indulges in sexual relations with a dog. Such men and women are put into the hell known as Vajrakaṇṭaka-śālmalī. The Kṛṣṇa consciousness movement forbids illicit sex. From the description of these verses, we can understand what an extremely sinful act illicit sex is. Sometimes people disbelieve these descriptions of hell, but whether one believes or not, everything must be carried out by the laws of nature, which no one can avoid.
(Srimad Bhagavatam----5:26:21----purport).

A person who kills animals in the name of religious sacrifice is put into the hell named Viśasana.
(Srimad Bhagavatam----5:26--summary).

A person who in this life is proud of his eminent position, and who heedlessly sacrifices animals simply for material prestige, is put into the hell called Viśasana after death. There the assistants of Yamarāja kill him after giving him unlimited pain.
(Srimad Bhagavatam----5:26:25----translation).