'Ridiculousness' Star Steelo Brim -- Investigated For Allegedly Sexually Assaulting Porn Star

MTV's "Ridiculousness" star Sterling "Steelo" Brim is accused of sexually assaulting a porn star in NYC, but claims it's BS, and she threatened to concoct a phony rape story ... TMZ has learned.

Law enforcement sources tell us Steelo and porn star Mary Jean were hanging out Friday night, but she claims things went south fast. Mary says she told cops ... while she was driving Steelo wanted sex, and when she refused he grabbed her crotch, punched her breasts, and said "this belongs to me."

Here's where the story gets really strange. Steelo says Mary refused to let him out of the car, and threatened to tell people he had raped her ... if he got out. Steelo says he weighed his options and decided to leap from the moving car. He says he tucked and rolled on the street ... injuring his arm and his Rolex.

Steelo denies assaulting Mary ... who went straight to police after he escaped, and reported the alleged sexual assault.

Mary denies blocking Steelo from getting out of the car, and says he could have fled at a stop light. She says she was trying to take him to police.

Police are investigating Mary Jean's claims. Steelo tells us he hasn't spoken to cops yet, and no charges have been filed.