But William Roberto Gulson told a court on Wednesday said he never believed he was messaging a child because their online profile on the adult site stated they were in their 20s.
Gulson taught at Sydney's elite Knox Grammar School and conducted private tutoring before he was charged over his messaging with a 15-year-old girl, posing as a boy, in September 2024.
The 15-year-old and another teenager were running a fake profile on LGBTQI dating app Grindr with the aim of catching pedophiles, the Downing Centre Local Court has been told.
The fake profile told Gulson they were in year 9 before sending Gulson a photo of an erect penis.
A later message stated their age was 15 years old.
"Wow, so young, I feel conflicted," Gulson responded.
But he testified that he never believed the user was a child.
"I believed I was dealing with an adult male that was using the site," Gulson said.
Under cross-examination, he said he believed the exchange was a bizarre attempt by a "wacko adult" to pretend they were a child.
That followed the prosecutor suggesting he had never checked whether the person was "pretending to be 15".
"Surely if there was even a risk you were speaking to a 15-year-old you should end the conversation," prosecutor Jessica Chan said.
Questioned about why Google searches for "gay child porn" and "young boy raped movie scene" appeared on data extracted from his phone, Gulson said he had never accessed illegal pornography.
"The vast majority of what (was) extracted has been legal pornographic content that I have accessed," he said.
Some of the search terms may have been "automatically entered" in his search bar, he suggested.
Gulson, supported by his brother, shook his head repeatedly through Ms Chan's submissions for why the case should continue.
The 28-year-old testified on Wednesday after his lawyer failed to get Gulson's single charge of procuring a child under 16 for unlawful sexual activity dismissed.
Gulson's lawyer Paul Glissan argued there was no case to answer because the children talking to the former teacher had no intention of meeting him.
"The child who was doing the chatting was actually enticing the defendant," Mr Glissan said.
Ms Chan said whether or not the children intended to meet Gulson was irrelevant and the charge only related to the conduct of the 28-year-old.
"A police officer posing as a child to catch pedophiles has no intention of meeting with them," she said.
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