Śrīmad-Bhāgavatam · Text 11.3.18
Who is speaking and story frame
श्रीप्रबुद्ध उवाच
कर्माण्यारभमाणानां दु:खहत्यै सुखाय च ।
पश्येत् पाकविपर्यासं मिथुनीचारिणां नृणाम् ॥ १८ ॥
śrī-prabuddha uvāca
karmāṇy ārabhamāṇānāṁ
duḥkha-hatyai sukhāya ca
paśyet pāka-viparyāsaṁ
mithunī-cāriṇāṁ nṛṇām
Śrī Prabuddha said: Accepting the roles of male and female in human society, the conditioned souls unite in sexual relationships. Thus they constantly make material endeavors to eliminate their unhappiness and unlimitedly increase their pleasure. But one should see that they inevitably achieve exactly the opposite result. In other words, their happiness inevitably vanishes, and as they grow older their material discomfort increases.
Read the purport on Vedabase.ioFrom Canto 10, chapter 14 onward, the translation and purports were completed by Śrīla Prabhupāda's disciples after his departure, and published by the Bhaktivedanta Book Trust.