67-year-old Gwyn Jones began punching the man and shouting threats and abuse at his wife, on Sutton Road, on August 31.
But the man hit him back and the pair fell to the floor, where Jones received injuries to his face and elbow, said prosecutor Donna Fawcett.
David Grant, mitigating, said: “He confronted them and threw a single blow. He sustained injuries and had to go to hospital. As far as he’s concerned the marriage is over.”
Jones confirmed to the court that he is divorcing his wife.
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Jones, of Market Street, admitted assault by beating and using threatening words and behaviour, when he appeared at Mansfield Magistrates Court, on Wednesday.
District judge Jonathan Taaffe said: “It’s sad when someone at your age of 67 comes before a court.
“To some extent I can understand your emotions on the 31st of August.
“I say to some extent because while it probably came as a blow to you to find that your wife thought the relationship was over and was moving in a different direction - your reaction was not acceptable.
“I accept that it’s out of character. Your wife has a right to do what she wishes to do and should not be subject to threats and violence.”
He handed Jones a two year conditional discharge, with £85 costs and a £20 government surcharge.
A three year restraining order, banning Jones from contacting his wife, except through a solicitor, was also made.