Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 11


तद्भवान्दह्यमानायां सपत्नीनां समृद्धिभि: ।प्रजावतीनां भद्रं ते मय्यायुङ्क्तामनुग्रहम् ॥ ११ ॥


tad bhavān dahyamānāyāṁsa-patnīnāṁ samṛddhibhiḥprajāvatīnāṁ bhadraṁ temayy āyuṅktām anugraham


Therefore you should be kind towards me by showing me complete mercy. I desire to have sons, and I am much distressed by seeing the opulence of my co-wives. By performing this act, you will become happy.


In Bhagavad-gītā sexual intercourse for begetting children is accepted as righteous. A person sexually inclined for simple sense gratification, however, is unrighteous. In Diti’s appeal to her husband for sex, it was not exactly that she was afflicted by sex desires, but she desired sons. Since she had no sons, she felt poorer than her co-wives. Therefore Kaśyapa was supposed to satisfy his bona fide wife.