Shape

Muh Sundae. MUH SUNDAE!

(To those that the meme went above their heads, it's from Game Of Thrones)

Prior to this chapter, our MCs were apparently unable to achieve full
penetration during sex (likely due to Wakaba's petite body). However,
their frustration and disappointment would make a rather unsatisfying
story, so a prequel probably doesn't exist---which makes this chapter
a good example of in medias res, for it begins in the middle of the story.
https://en.wikipedia.org/wiki/In medias res

That's what I was thinking, too!