Sean Miller (29) trashed the taxi and punched the driver (70) to the ground, leaving him black and blue and "pumping blood" on a south Dublin street.
The gay couple had been returning home from a night out when Miller took offence at the alleged remark about their sexuality and set upon the shocked driver.
Miller was found guilty of assaulting the victim, Cathal Hawkes, and causing over €1,500 of damage to his car.
Dublin District Court heard neither Miller nor his partner, Niall Byrne, could elaborate on their claim of an alleged offensive remark which they said prompted Mr Byrne to get out of the moving cab.
The pair had each downed between six and eight vodkas at a nightclub earlier.
The victim denied making any derogatory remark at all and Judge Catherine Murphy said she found the accused's evidence "lacking in credibility".
She adjourned the case for sentencing at a later date.
Miller, of Mount Anville Park, Goatstown, had pleaded not guilty to assault causing harm to Mr Hawkes and criminal damage at Northumberland Road.
The victim said he had picked up Miller and Mr Byrne at Nassau Street at 3am on December 14, 2011.
On the way to their home at Elm Park Apartments, he said, the accused made a statement to his partner, telling him: "I am not a c***", and repeating it.
Miller then leaned across to Mr Byrne, put his arm around him and said: "You know I love you." Mr Byrne suddenly opened the door while the car was moving and got out. The accused then threw a ¤5 note into the cab.
The fare was more, but Mr Hawkes told Miller to go on because he was "glad to get rid of them". Miller kicked the front passenger door, jumped across the bonnet and flattened the BMW sign before punching the driver's window and door.
Mr Hawkes jumped out and said: "calm down" but Miller punched him "a ferocious belt" to the side of his face.
He fell to the ground "pumping blood" and Miller walked away. Judge Murphy adjourned the case for a week.