Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 33

Sanskrit:

दितिस्तु व्रीडिता तेन कर्मावद्येन भारत ।उपसङ्गम्य विप्रर्षिमधोमुख्यभ्यभाषत ॥ ३३ ॥

ITRANS:

ditis tu vrīḍitā tenakarmāvadyena bhārataupasaṅgamya viprarṣimadho-mukhy abhyabhāṣata

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.