The victim, Joseph Hossell, had slapped the woman’s rear in an address in Morecambe.
Her boyfriend Daniel Mason, 28, of Euston Road, Morecambe, Lancashire, and his friend took exception to the act and attacked him, Preston’s Sessions House Court was told.
The incident happened on November 26, 2016.
Mason and pal Michael Davies, 29, of Blea Tarn Place, Morecambe, could now face a jail term, a judge has warned.
Prosecuting, Stephen Parker said: “The girlfriend of Mason was in the living room. As she walked past Mr Hossell he tapped her bottom. These two defendants took exception to that.
“There were numerous punches thrown, leading to his facial injuries.”
Both men pleaded not guilty to wounding with intent but admitted a lesser charge of wounding.
Adjourning the case for reports, Judge Simon Newell said: “You have to be aware custody will be at the forefront of the court’s mind.”