While I can understand your point, I am feeling as if you are seeing things through a lack of consideration for the sake of simplicity. And yes, the mangaka can do whatever the fuck they want with their manga, it is their manga, lol. They can randomly shift it into an isekai where Akita is suddenly genderbent. However, the fact remains that the chapter is specifically an extra chapter, posing no true relevance to the main story. It is quite literally a wet dream Akita so happened to have. It was a slice of life moment basically.
Some more points I would like to make is that I don't even really understand how the manga was given the erotica tag in the first place. There is no actual nipples shown besides in the extra chapter. One Piece and Bleach are good examples. They have a ton of fanservice, especially in one of the chapter's of bleach where they quite literally show Yoruichi’s ass in the first panel and it is only given the suggestive tag. There is even a fight scene where Yoruichi is basically wearing a g-string made of lightning. The fact is, Bleach and One Piece are suggestive at best. Hell, I can get a little bold and bring in Berserk as an example because it literally shows a white length of a horse in one of the chapters and it is not given the mature tag.
Giving this manga a mature tag also deturs those away from the manga, expecially for those who are not interested in blatant pornography, which a lot of doujinishi is on Mangadex. They are blatant porn, which this manga is not, however can confuse future readers. Best case, Mangadex makes a fifth tag that is stated as explicit pornography, because yeah, this manga is for mature audiences, but these terms have different meanings under different contexts, and this manga, however mature, is not blatant pornography that most mature mangas on Mangadex actually point out.