The prostitution tag should 100% be on this chapter. It's not technically prostitution in the sense that the male actually paid for sex, per se, but it might as well be.
"There are (figuratively or literally) Eight Million Stories in the Universe of Hentai. This is simply one of them."
Guess one could conclude that this session escalated unexpectedly. It's almost anyone's guess what may transpire in their next appointment ... unless Kazuki-sensei drops a continuation story in that regard on us one fine day.