This was surprising, but not unwelcome for me. I actually read this story for the plot, not the "plot" so I liked this chapter because it was entertaining for me. To be honest, I've seen better sex based stories in real ero-manga. I forget the name of it, but there was one with a guy who moved in with a package delivery girl while going through a messy divorce with his now ex-wife. That one was riveting, although much less for its sex scenes than the character and story development.
As for just raw sexual content, the artist of this work does that better in his own actual h-manga.