Murphy and Lisanne, an events planner, are on trial for outraging public decency at the concert on September 14 last year, which was watched by 1.2 million people on TV and featured acts including Jeff Lynne's ELO, Chrissie Hynde, Blondie and Billy Ocean.
But William Saunders, defending, said even if the sex act did happen, it had raised a laugh in court and could not be considered "outrageous".
"Let's think actually there was sucking the penis, nobody is condoning it," he told the jury.
"Of course it must have upset the sensibilities of some there, it must have caused annoyance.
"But it is my submission that it did not outrage public decency.
"I don't think anyone in this case - be they the person that took the footage, witnesses, yourselves, others in court - not a person in this case has at some stage failed to see the funny side.
"We have had some chuckles in this court - I absolutely welcome it - it tells the story.
"Actually this isn't outraging public decency at all, it doesn't go hand in hand with there being a funny side."
The jury have been shown mobile phone footage of the couple lying on the grass at the time they were alleged to have been engaged in the sex act.
Beck, wearing a red vest top and white shorts, can be seen moving her head towards Murphy, dressed in a purple T-shirt and khaki shorts.
"The gentleman was lying apparently asleep or drunk on his left hand side and it came to our attention that the lady seemed to be what appeared to be in a drunken manner trying to undo the gentleman's shorts," said prosecutor Tom Cleeve.
"It became apparent she seemed to be undoing the gentleman's shorts and trying to release the button fly on the shorts.
"She appeared to manipulate the gentleman's genitals."
Murphy initially told police it was a case of mistaken identity, but now claims he was unconscious and unaware what was happening.
"You have had a few drinks, it's a very nice afternoon, it's plainly warm and you have been up for a very long time," said Mr Saunders.
"There's some nice soft grass and some music in the background - to crash out you would possibly want to do and that's exactly what Mr Murphy chose to do.
"He had no idea about what was going on - you can see him crashed out in that film, can we not?"
Murphy did not give evidence to the trial, but Mr Saunders said that was understandable.
"He was out of it on the ground, that's why he had the shock of his life when he was hauled to his feet, marched out, arrested, and accused of being involved in this sex act," he added.
"To be guilty, you have to know what is going on, but lying comatose on the ground is very much otherwise."
Beck, who did give evidence, claimed she had been trying to shake her partner awake during the incident.
Beck and Murphy, of Mumbles, Swansea, deny outraging public decency on 14 September last year.
The trial continues.
