The way we define Hentai is a focus on the sexual act, and depiction of genitals.
Which this manga clearly fulfills.
And besides that, there are other reasons that explicit content has to be marked and hidden away behind the hentai toggle.
Certain countries like Indonesia (which is our 2nd highest source of traffic) ban websites with explicit content, our .com domain was already banned there because of that.
After implementing the toggle in the user settings (and switching to the new domain, which we only did to circumvent the first ban) we have been safe, and we don't want to risk another ban just because this manga isn't published in a hentai mag.