'Dr. 90210' Robert Rey -- I Punched a Guy ... For Squeezing My Nuts

"Dr. 90210" Robert Rey claims he was assaulted by a drunk dude in Brazil this weekend ... and the guy grabbed him by the nuts so hard the reality star says his testicles are STILL inflamed.

The doctor was dining at a restaurant Saturday in Campina Grande when he claims a man asked him to snap a pic. Rey obliged, but the man groped his crotch out of nowhere ... calling him "Dr. Gay."

Rey, who studies martial arts, says he defended himself by punching the guy twice in the jaw, and then took him down with a kick. His scrap with the guy sparked a drunken brawl between several other people in the restaurant, but Rey stayed out of it.

The reality doc says he didn't call cops and doesn't want to press charges because, "I was young and also did youth nonsense!"

Rey posted a picture of the nut job to prove he was justified in defending his family jewels.