"It's the worst thing to ever happen to me," the man, who cannot be identified for legal reasons, told Sydney's Downing Centre District Court on Friday.
"You don't think that, as a kid, a public figure looked up to by other people is going to exploit you."
Orkopoulos, 65, has pleaded not guilty to 28 charges, including sex offences against underage boys that allegedly occurred between 1993 and 2003.
He was the member for the NSW seat of Swansea from March 1999 until November 2006, when he was booted from cabinet and expelled from the Labor Party.
Orkopoulos faces eight counts of supplying a prohibited drug, eight counts of indecent assault and 11 counts of sexual intercourse with a person aged between 10 and 16.
He is also accused of perverting the course of justice after allegedly having one complainant sign a statement to retract an allegation.
The court heard allegations Orkopoulos assaulted one complainant at least four times.
In one instance, Orkopoulos asked him if he wanted money, the man said in a statement to police that was read in court by defence lawyer Paul Johnson.
Orkopoulos allegedly gave him $30 and a pack of cigarettes after he forced the then 13-year-old boy to perform a sex act on him in his car.
"Can you give any reason why you're being friendly to him still (after being assaulted previously)?" Mr Johnson asked the man on Friday.
The man replied: "I did think he was there to help. He was a public figure that was meant to be trusted."
Orkopoulos was elected as the Swansea MP the same year the man began high school.
The man's mother said her son's behaviour began to deteriorate in year 7.
"He was caring, he loved to skateboard and ride bikes and be outside. He loved to go fishing. He loved to go to his grandmother's. He liked to cook," the mother told the court on Friday.
"He started to truant (in year 7) and he ran away once."
The man is one of four complainants giving evidence at the trial.
Three complainants were part of the same group of friends and one of them first met Orkopoulos, then on the local council, during the lobbying for a new skate park, crown prosecutor Cate Dodds said.
"Some of them come from very broken homes," Ms Dodds said at the trial's opening in early March.
"The accused took advantage of every single one of them."
The trial continues.
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