John Parker punched Adam Metcalfe after finding him sitting on the sofa of his former marital home, drinking a glass of wine, the court heard on Friday.
Parker, who had separated from his wife, had gone to get medicine for his child, who was staying with him for the weekend, the court heard.
Parker, who admitted assault by beating at the address in Low Bentham Road on February 18, had made a gross error of judgement, the court was told.
In mitigation, Paul Fitzpatrick said the self-employed agricultural worker had put two and two together and come up with five after finding Mr Metcalfe, a man he knew, with his wife.
Parker, of Low Bentham Road, was fined £150 and ordered to pay costs of £85 and victims surcharge of £20.
He was also ordered to pay compensation of £75 to Mr Metcalfe, who suffered a bloodied nose and sore lip.