SB 11.3.18

śrī-prabuddha uvāca
karmāṇy ārabhamāṇānāṁ
 duḥkha-hatyai sukhāya ca
paśyet pāka-viparyāsaṁ
 mithunī-cāriṇāṁ nṛṇām
Word for word: 
śrī-prabuddhaḥ uvāca — Śrī Prabuddha said; karmāṇi — fruitive activities; ārabhamāṇānām — making endeavors in; duḥkha-hatyai — for the elimination of distress; sukhāya ca — and for gaining happiness; paśyet — one should see; pāka — of the result; viparyāsam — contrary outcome; mithunī-cāriṇām — who are coupled as men and women; nṛṇām — of such persons.
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.