The 83-year-old from Dalston and an Aspatria man, 70, were caught on CCTV camera in the bedding section of Debenhams in Carlisle city centre.
It is not clear how many customers witnessed the pair, who continued their sexual activity in the toilet, where they were tracked by security staff.
Their arrest on suspicion of gross indecency led to a police caution.
The incident took place on Saturday lunchtime.
Insp Paul Coulson, of Cumbria Police, said: "In my 25 years of police service, it is one of the most unusual incidents I have come across."
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