SB 5.26.20
yas tv iha va agamyam striyam agamyam va purusam yosid abhigacchati tav amutra kasaya tadayantas
tigmaya surmya lohamayya purusam alingayanti striyam ca purusa-rupaya surmya.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 Yamaraja in the hell known as Taptasurmi. 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.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.