MELISSA RIVERS Dating XXX Porn King

Joan Rivers' only daughter Melissa has been meeting with the king of celebrity sex tapes Steven Hirsch ... but it ain't to star in a XXX flick, TMZ has learned the two are actually dating.

Sources close to the couple tell us Melissa -- who runs "Fashion Police" on E! -- has been dating the Vivid Entertainment honcho for a couple of weeks ... after meeting through mutual friends.

One person with direct knowledge of the situation tells us, "The relationship is going great."

Hirsch has brokered nearly every major celebrity sex tape on the market ... including Kim Kardashian, Shauna Sand and Chyna ... but he's never starred in one ... so don't get your hopes up.