It was ON SIGHT when openly gay boxer Yusef Mack caught up with a man in a Philly barbershop, who had been trolling him online over his sexuality. Via TMZ: TMZ Sports confirmed paramedics were called to the L A Clippers shop in Philadelphia Saturday evening ... after Mack wailed on the man in front of customers and employees. Witnesses tell us Mack attacked the dude while the victim's head was in a sink in the middle of a wash. Mack tells us he got sick and tired of the

online harassment and decided to confront the guy face-to-face. Mack is unapologetic and says the guy got what he deserved. We also reached out to the victim who didn't want to talk ... but has since issued several homophobic statements on his Facebook page, including: "I rather have a video of some beating me up then a video of someone seeing me take a d*** up my a** any day." The man says he will retaliate against Mack ... "Me and him gonna fight again." Watch the beatdown below: