Lisanne Beck, 47, allegedly performed a sex act on Simon Murphy, 48, during a Paloma Faith set in September.
The Old Bailey heard the couple were filmed conducting the alleged behaviour, but the jury failed to reach a verdict and were discharged.
The pair, from Swansea, denied outraging public decency.
Judge Peter Rook QC has given the prosecution until Thursday to decide whether to pursue a retrial.
He said: "Whatever the Crown says, it will have been a chastening experience over the last few days, I'm sure everyone will agree with that."
During the hearing prosecutor Tom Cleeve told jurors while "there are some amusing aspects to it... it was not funny at the time" and a 16-year-old girl had to be shielded from what was going on.
He said the two defendants were of "previously good character, but in that afternoon I'm afraid they blotted their copybook".
During the hearing Ms Beck said the film showed she was trying to wake her boyfriend up, while Mr Murphy's legal team said he was completely unaware of what his girlfriend was doing.

The jury praised Ahmed Mansoor for regularly raising concerns about arbitrary detention, torture and degrading treatment in the Gulf state.
As a result, it said, he had faced repeated intimidation, harassment and death threats from the UAE authorities and their supporters.
Mr Mansoor could not collect the award in Geneva because of a travel ban.
The Martin Ennals Award is named after the late British lawyer who became the first head of the human rights organisation, Amnesty International.
Mr Mansoor was selected as one of the human rights defenders nominated for the award by a jury of 10 global human rights organisations. The other nominees were Asmaou Diallo of Guinea and Robert Sann Aung of Myanmar.
Before Tuesday's awards ceremony, the jury had said Mr Mansoor's absence would "mark a very disappointing position for the UAE, which is a country that prides itself as one of the hubs of international business and tourism in the Middle East, as well a safe haven in the region".
"As a member of the UN Human Rights Council, which is running for a second term, we expect the UAE authorities to honour their obligations to uphold human rights and protect human rights defenders," a statement said.
Mr Mansoor and four other activists who called for democratic rights in the UAE were arrested and jailed in 2011 on the charge of "insulting officials".
Although pardoned and released later that year, he was banned from travel and had his passport confiscated.
In a video message recorded for the ceremony, Mr Mansoor said: "I hope that my presence as a finalist can show people that we have deep human rights issues that PR companies and a complex network of interests try to dismiss, or at least present as contrary to the reality."
The UAE has shown little tolerance towards dissent since the 2011 Arab Spring. Dozens of people have been detained and most have been tried and convicted of planning to overthrow the government.
A couple accused of carrying out a sex act at a BBC Radio 2 concert in Hyde Park must wait to hear if they face a retrial after the jury was discharged.
[NEXT_CONCEPT]
An activist from the United Arab Emirates has won the Martin Ennals Award for Human Rights Defenders.