Lisanne Beck, 47, gave her partner Simon Murphy oral sex during a BBC radio 2 concert - despite it taking place in one of the capital's busiest greens.
As Paloma Faith sang on stage the couple got over amorous - with one shocked onlooker filming the event on his phone.
'I was repulsed and very upset by it' Mike Holden told the Old Bailey.
'It was a family event and there were children in the vicinity.'
However, despite the damning video footage, Beck claims she was merely trying to 'shake' her boyfriend awake after he had fallen asleep on the grass, and she is bewildered by the sex claims.
Prosecutor Tom Cleeve said back: 'You shook his legs, that didn't work - what else did you shake?'
The couple stand accused of outraging public decency on September 14, 2014.
The trial continues.