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Bg 18.38

viṣayendriya-saṁyogād
yat tad agre ’mṛtopamam
pariṇāme viṣam iva
tat sukhaṁ rājasaṁ smṛtam
Synonyms: 
viṣaya — of the objects of the senses; indriya — and the senses; saṁyogāt — from the combination; yat — which; tat — that; agre — in the beginning; amṛta-upamam — just like nectar; pariṇāme — at the end; viṣam iva — like poison; tat — that; sukham — happiness; rājasam — in the mode of passion; smṛtam — is considered.
Translation: 
That happiness which is derived from contact of the senses with their objects and which appears like nectar at first but poison at the end is said to be of the nature of passion.
Purport: 

A young man and a young woman meet, and the senses drive the young man to see her, to touch her and to have sexual intercourse. In the beginning this may be very pleasing to the senses, but at the end, or after some time, it becomes just like poison. They are separated or there is divorce, there is lamentation, there is sorrow, etc. Such happiness is always in the mode of passion. Happiness derived from a combination of the senses and the sense objects is always a cause of distress and should be avoided by all means.