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.
Generally a man should not have sexual relations with any woman other than his wife. According to Vedic principles, the wife of another man is considered one’s mother, and sexual relations are strictly forbidden with one’s mother, sister and daughter. If one indulges in illicit sexual relations with another man’s wife, that activity is considered identical with having sex with one’s mother. This act is most sinful. The same principle holds for a woman also; if she enjoys sex with a man other than her husband, the act is tantamount to having sexual relations with her father or son. Illicit sex life is always forbidden, and any man or woman who indulges in it is punished in the manner described in this verse.
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.
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.
Source: Srimad Bhagavatam 5.26.20-21