Bg 18.38

visayendriya-samyogad
yat tad agre ’mrtopamam
pariname visam iva
tat sukham rajasam smrtam

Word for word: 
visaya — of the objects of the senses; indriya — and the senses; samyogat — from the combination; yat — which; tat — that; agre — in the beginning; amrta-upamam — just like nectar; pariname — at the end; visam iva — like poison; tat — that; sukham — happiness; rajasam — in the mode of passion; smrtam — is considered.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.