Muh Themes...

Anything that doesn't serve the plot is self-indulgent and gratuitous sex scenes are a prime example.

There are many things you can do that aren't directly related to plot that still serve it (character development, world building, misdirects etc). But there's a simple question you can ask yourself to determine of something is gratuitous: if I took this scene out, would any other scene be less impactful?

Unless the movie is about the way in which sex is had, the mechanical act is gratuitous.