Mihail Iliev, 32, had only arrived in Scotland from Bulgaria six weeks before the incident which put him in the dock.
Iliev had been in Coronation Park in Port Glasgow, Inverclyde, with a friend when they saw a girl aged 14, with her boyfriend, aged 15.
He told the court he had seen the teens engaged in a sex act, and had wanted to confront them, getting his mobile phone out to film what was happening.
He followed the girl into a public toilet and opened the cubicle door while she was inside, but claimed he thought the teens were 'behaving abnormally'.
Iliev claimed to believe the young couple were either hiding a dead body, or having sex.
He went on trial at Greenock Sheriff Court accused of sexual assault and behaving in a threatening or abusive manner likely to cause a reasonable person to suffer fear or alarm by entering the female toilets and opening a cubicle door.
But a sheriff found the sex assault charge not proven, and convicted him of the second charge.
Iliev told the court he had seen the teens engaged in a sex act, and had wanted to confront them, getting his mobile phone out to film what was happening.
Speaking through an interpreter, Iliev said: "I wanted to take footage.
"I saw them in the middle of a sexual act.
"I started talking in Bulgarian, saying, 'Shame on you, this is a public place'."
However, he told the court that his phone battery had died.
Iliev insisted the girl had been 'flirting' with him as he worked out in the park, and it made him 'embarrassed'.
But the teenage girl claimed that Iliev grabbed her bottom, on May 22.
Her boyfriend gave evidence and said he had heard her 'screaming' while he was in the men's toilets.
Forensic officers could find no trace of Iliev's DNA on her clothing.
The girl's boyfriend's mum told the court the teenager was 'bubbly and outgoing' but after the encounter with Iliev was 'chalk white and visibly shaking'.
Fiscal depute Frankie Morgan questioned whether Iliev had 'taken an opportunity' when he saw the girl go into the toilet.
Mr Morgan said: "You say you thought at one point that they were doing something with a dead body."
Iliev said: "They were in and out [of the toilet], how would I know what they were doing?"
Sentence was deferred until October 1.
Sheriff Kelly said: "I want to be addressed on any significant sexual element insofar as it relates to this offence."