Historic Scotland, which owns Glasgow Cathedral where the encounter was said to have taken place, held an inquiry into allegations of misconduct by the woman in her thirties relating to an incident with choirmaster Iain Simcock. It found no evidence to support the claims.
Mr Simcock, 47, was sacked from his role as director of music at the cathedral after a separate investigation by the Church of Scotland into the allegations.
The couple did not deny being together on the lawn but rejected the sexual nature of the charges which had been described by the Church of Scotland in a letter of termination in June.
The choirmaster now claims to have been vindicated after his girlfriend was cleared over the issue and has asked the Kirk to apologise.
Historic Scotland, which also stewards the 800-year-old cathedral, completed its inquiry last week and Mr Simcock's girlfriend continues in her job.
Mr Simcock claimed the outcome of the latest probe confirmed "that the disgraceful allegations are baseless".
He said: "The Church of Scotland has acted appallingly and I question its claim to being a responsible employer. We found the allegations repulsive and so foreign to our natures, that we feel deeply damaged. We hope that time's healing powers will not leave any long-term consequences for us both.
"We will not rest until the Church of Scotland and Glasgow Cathedral write us a letter of apology, the only way it can hope to regain any credibility and dignity as an employer, let alone a Church."
Mr Simcock's girlfriend, the identity of whom The Herald has not disclosed, declined to comment.
Former Westminster Abbey organist Mr Simcock said he has been the victim of character assassination.
The choirmaster, who is also music director at the Paris Academy of Voice in France, was dismissed by the Kirk for what he insisted amounted to "a kiss and a cuddle".
In his letter of termination, Mhairi Coutts of the cathedral's music committee wrote that Mr Simcock had been "observed in the Graveyard of Glasgow Cathedral partially dressed and having sexual relations with a female."
This was said to be "wholly unacceptable, brings the cathedral into disrepute and falls far below the standard that the cathedral is entitled to expect from its employees".
One of the two witnesses described as independent by the Kirk was a gardener working for Glasgow City Council. The council declined to comment.
During a controversial tenure as choirmaster a number of members left the singing group claiming he had bullied them, allegations which he denied.
Mr Simcock said that when he took up his position there was already discontent at the choir and the cathedral.
A spokesman for Historic Scotland, said: "We have concluded our internal investigation into the allegations of misconduct at Glasgow Cathedral. There was no evidence to support the allegations and we have drawn a line under the matter."
A spokesman for the Church of Scotland said: "Mr Simcock was on a probationary period as director of music at Glasgow Cathedral.
"The cathedral was not satisfied with his performance in the post, and he had been warned about his behaviour.
"Specifically he had been told that if cause for further disquiet came to light, the Kirk Session would be left with no option but to dismiss him.
"The cathedral felt that the incident which occurred towards the end of May this year was such a cause for disquiet."