In Once Upon a Time in Hollywood, when Sharon Tate and her friends are arriving at El Coyote towards the end of the movie, there is a crowd noise coming from a pornographic theater and Sharon Tate remarks something along the lines of "They're making opening nights for porn movies now?". Jay Sebring replies "Yes and they're fun!"
I feel like I am missing a reference here, is it a nod to some particular event?
It is not a reference to the movie, but the theater which was famous for playing adult movies at that time. This theater is New Beverly theater previously known as Eros Theater and is owned by Quentin Tarantino.
From GameSport article,
Hollywood is as much a character in this movie as Rick or Cliff, and many of the locations visited in the film are real historical restaurants and theaters, including Musso and Frank Grill, El Coyote, the Pacific Cinerama Dome, Pantages Theater, Supply Sergeant, Chili John's, the New Beverly theater (which Tarantino owns, and which used to be a porno theater, as it appears in the film), the Fox Westwood Theater, and of course the early Taco Bell and Wienerschnitzel restaurants. Most of these are still around today, as well.
From Business Insider,
The night of the murders, Tate and her friends went to dinner at a Mexican restaurant called El Coyote. In the movie, when she walks into the restaurant, Tate notices a porn movie premiere happening down the street. This was likely happening at the Eros Theatre, which, at the time, was an adult movie theater just around the corner from El Coyote. Today, the Eros Theatre is the New Beverly Cinema, a movie theater owned by none other than Quentin Tarantino.
That might be a nod to Leonardo di Caprio turning down a role of Dirk Diggler in Boogie Nights, a movie about porn industry where they made "first real movie".
