SB 3.14.33

ditis tu vrīḍitā tena
 karmāvadyena bhārata
upasaṅgamya viprarṣim
 adho-mukhy abhyabhāṣata
Synonyms: 
ditiḥ — Diti, the wife of Kaśyapa; tu — but; vrīḍitā — ashamed; tena — by that; karma — act; avadyena — faulty; bhārata — O son of the Bharata family; upasaṅgamya — going nearer to; vipra-ṛṣim — the brāhmaṇa sage; adhaḥ-mukhī — with her face lowered; abhyabhāṣata — politely said.
Translation: 
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: 

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.