Rory Amon was a 27-year-old aspiring politician when he met the teenager at his Sydney apartment block in July 2017.
He admits having sex with the 13-year-old on one occasion at night in the underground car park bathroom.
But this was far from a case of a man lurking in a space for children, a NSW Supreme Court jury was told on Tuesday.
Amon met the teen on a gay dating website where users had to be over 18 to sign up, defence barrister Matthew Johnston SC said.
"This is not a case ... where there is an adult who's on a chatroom that's frequented by 13-year-olds and is tricking them into a meeting," he told the jury.
The alleged victim admitted lying about his age, once in signing up to the website and again when chatting to Amon, Mr Johnston said.
This was the only reason his client had met the boy, he argued.
If the teen had said he was 15 - as he claimed in the witness box - the former politician would have ceased chatting to him immediately, the barrister said.
Instead, the 13-year-old bolstered his lies by sending photos and messages, which made him seem older with the intent of meeting Amon for sex, Mr Johnston said.
"I'm raising that not to, in any way, say that (the alleged victim's) to blame for anything," he told the jury.
"He was 13 ... adolescents make stupid decisions all the time."
But the jury was urged by prosecutors to reject Amon's evidence he thought the teen was above the age of consent.
"Using your common sense and experience, you may have very little difficulty finding this accused is guilty," crown prosecutor Meaghan Fleeton said.
It would have been obvious to Amon when he met the boy in person he was under 16, she argued.
She said the intelligent, articulate former MP had lied about his age to manipulate the boy before pursuing and sexually assaulting him.
The alleged victim was a child with no sexual experience at the time, she said.
Now 36, Amon has pleaded not guilty to 10 child sexual abuse, child rape and indecent assault charges.
He admits meeting the boy for sex once, but denies allegations of a second meet-up where a towel was brought to the bathroom.
Mr Johnston suggested the incident had "grown" in the complainant's mind over the years as he grappled with its impact.
"'Some witnesses - they believe they're telling the truth ... but they're genuinely mistaken about some things," he said.
But the Crown portrayed the key witness as honest and who broke down when testifying.
"You saw (him) get upset when shown the photos of the car park toilet - that dirty, hidden car park toilet - where he had his first sexual experience," Ms Fleeton said.
"It was a significant and life-changing course of events."
There would have been a window of opportunity in July 2017 for both of these "hasty hook-ups" to have taken place, she argued.
Soon after the alleged sexual abuse, he gave some details of what occurred to his mother, friends, school counsellor and psychologist, she noted.
The complainant first became aware of Amon's identity in 2020 through a COVID-19 announcement when the former Liberal was a councillor.
The two reconnected on Snapchat in 2019 and then on Grindr in 2022 when Amon sent him some photos, including one in his volunteer firefighter uniform.
The trial continues on Wednesday.
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