This all went down at Christie's Toy Box in Oklahoma City ... where Pottawatomie Sheriff's commander David DeWitt took his wife shopping for a new sex toy. According to the arrest report, obtained by FOX25, DeWitt grew enraged Monday night when the missus picked up a few items that were, umm ... "bigger than him."

The clerk says DeWitt told his wife she didn't need fake penises that put him to shame, and allegedly raised his hand as if to backhand her. The clerk says he stepped in between them at the point and asked DeWitt to leave the store, but he allegedly barked back, "F*** you, I'm a cop!"

The violence allegedly broke out when DeWitt and his wife were checking out, and that same clerk asked if they wanted to buy any batteries for their new device. That's when DeWitt allegedly yelled, "What the f*** you say to me, fat boy?"

The clerk says he told DeWitt to get out of his face, and that's when DeWitt jumped behind the counter and repeatedly punched him in the face and ribs, tossed a bunch of merchandise around the store ... and then left in a convertible Bentley.

Shortly thereafter, DeWitt was pulled over, but not for the sex shop incident -- a state trooper nailed him for speeding. The trooper was going to let DeWitt go with a warning, but then heard the call about the alleged fight over a phallus ... and notified OKC cops he had DeWitt pulled over.

DeWitt was booked for misdemeanor assault, and he's been suspended without pay pending an investigation.