I honestly can't think of any sex scene that was actually necessary

I generally agree.

I don't even think there was an explicit sex scene, but they did use sexy scenes to set some of the back story in The Nice Guys, where one of the characters was a porn star.

Other than that, occasionally sex scenes set the stage well if the tone of the movie is strange enough. Ninth Gate (by pedophile satanist whatever Roman Polanski) had a sex scene that worked within the film due to the overall tone.

Basically, if a movie is going for a specific weird tone, sometimes sex scenes fit that tone but, in general, sex scenes are completely unnecessary and usually counterproductive cringe.