Why do people suddenly care about the plot when sex scenes occur?

There seems to be an increasing sentiment across Reddit that whatever is in a movie NEEDS to be in there to further the plot of a movie. Yet, this sentiment always seems to pop up whenever sex scenes get mentioned. Do sex scenes need to be in movies? Perhaps not, but does anything need to happen in a movie?

Here's an example: do Dread Pirate Roberts and Inigo Montoya have to fight in The Princess Bride (1989)? After all, Roberts could have easily knocked Montoya out as soon as he got up from the cliff. Yet the movie would lose out on not only one of the greatest fight scenes of all time but a decent bit of depth from the involved characters.

The point is: the "plot" in a movie, much like a book, play, or even an album, is determined based on whatever the filmmakers want to show. Sex happens; therefore, there will be depictions of it whether as a detail or as a major part of the story. I understand there will always be people who aren't interested in sex in general and that's fine, but it's strange that those particular audiences don't do the bare minimum and check if the movie they're interested in has a sex scene or not.

As for the all-mighty plot worshippers, I wonder what their opinion is on movies such as The Thing (1982), Blade Runner 2049 (2017) , or even Solaris (1972) where all three are nearly devoid of sex (2049 does have nudity though), but are more than just a plot on a screen. Or even movies such as Oldboy (2003), Videodrome (1983), and Crash (1996) where sex is actually very important to the plot (that being said, in all three examples the audience is supposed to be uncomfortable).

Thoughts?