Srimad Bhagavatam Canto 03, Chapter 14, Text 33

SB 3.14.33

ditis tu vridita tena
 karmavadyena bharata
upasangamya viprarsim
 adho-mukhy abhyabhasata
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
O son of the Bharata family, Diti, after this, went nearer to her husband, her face lowered because of her faulty action. She spoke as follows.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
When one is ashamed of an abominable action, one naturally becomes down-faced. Diti came to her senses after the abominable sexual intercourse with her husband. Such sexual intercourse is condemned as prostitution. In other words, sex life with one’s wife is equal to prostitution if the regulations are not properly followed.
Srimad Bhagavatam Canto 03, Chapter 14, Text 32
Srimad Bhagavatam Canto 03, Chapter 14, Text 34