Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 18

Sanskrit:

श्रीप्रबुद्ध उवाचकर्माण्यारभमाणानां दु:खहत्यै सुखाय च ।पश्येत् पाकविपर्यासं मिथुनीचारिणां नृणाम् ॥ १८ ॥

ITRANS:

śrī-prabuddha uvācakarmāṇy ārabhamāṇānāṁduḥkha-hatyai sukhāya capaśyet pāka-viparyāsaṁmithunī-cāriṇāṁ nṛṇām

Translation:

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.

Purport:

Without the mercy of a pure devotee it is exceedingly difficult to free oneself from the bodily concept of life, which is the illusory basis of sexual attraction.