Buergrin pleaded not guilty and his lawyer argued that he was tricked by the 17-year-old girl and her pimp about her age.
The judge didn't think that was a defense, the report said.
The ex-banker, who was married with two kids at the time of the encounters, could be sentenced up to seven years and fined.
Fifty-one other men, both foreign and local, have been charged with having sex with an underage prostitute. Buergrin was the first to go to trial, though.