But Polwarth claims he never knew the boy’s real age was not 18 – and that he only learned the truth when he was back home in South Korea, where he taught at the time.
He was arrested at Glasgow Airport after jetting back from the East Asian nation, and had his passport confiscated as he was deemed a flight risk.
And, on Wednesday, he told his trial that he had been impacted physically, mentally and financially as a result of the one night stand.
Polwarth, who now works as a restaurant manager, said he lost his house car and home as a result.
He also said that it took him “a few days” to fully understand the situation and had to “seek help.”
And he said that, despite having agreed with Police Scotland that he would hand himself in when he returned, officers “broke that agreement” and arrested him at Glasgow Airport.
He explained: “I was deemed to be a flight risk and they said I needed to spend a night in the cells.
“I had to surrender my passport. I had to resign from my job as I was not allowed to leave the country.
“I lost a lot of money and a lot of things.
“I sought medical help in Korea straight away and was prescribed anti-depressants and some anti anxiety and sleeping tablets. I’m on anti-depressants to this day.”
When asked how he was at the time of the tryst, compared to now, he said: “It’s severely affected me.”
When prosecutor Ed Sheeran urged Polwarth to admit that he knew that the boy was 13, and had told him to delete their messages to cover up their illegal tryst, he replied: “I believed he was 18.
“I didn’t want him to be caught out – it was to prevent himself from outing himself.”
Polworth denies a charge of having sex with “a child who had attained the age of 13 years but had not attained the age of 16 years” in breach of Section 30 of the Sexual Offences (Scotland) Act 2009.
And he denies breaching Section 34(1) of the same Act by sending WhatsApp messages “of a sexual and indecent nature” to “a child who had attained the age of 13 years but had not attained the age of 16 years for the purposes of obtaining sexual gratification.”
The trial, before Sheriff James Spy, continues.