This was really a nice experience from those mangas with third wheeling character who just pop out of nowhere. I hope it remains like this and doesn't get any sort of drama that could ruin our fun. I loved it~! ♡
I have no idea why this is considered smut. It's entirely wholesome. There's a couple pages of drawn sex and those pages aren't even super sexualized either. That is not smut. I guess we're at a limit of the current tags on the site. I'd be hard pressed to even say it deserves an ecchi tag given how little screen time anything "perverted" is displayed.
The reason is, quite simply, because Mangadex never made, nor was inclined to make (for whatever reason), a "mature" tag or flag (such as Batoto or MangaUpdates, where the original tag set from, had)....
...and when people then mis-appropriated "smut" to cover for that, to mean "contains mature content of a sexual nature", the admins said, "yeah sure sounds good". So, here we are. The smut tag on Mangadex does not, in fact, indicate smut.
My (unfounded) suspicion is that the sort of manga the site admins usually read didn't use the tag much and they didn't really get what the problem was with this redefinition XD
Ecchi gets used for panty-flashing and moderately lewd flirtation... There is a huge difference for a good portion of readers between that and this sort of "by the way they have sex" develoment, even if the sex-scenes are fairly tasteful and tame (and in fact may fall below the usual bar for ecchi as far as erotic intent goes). That's why it was the "mature" tag, not the "oh no it's filthy" tag XD
Anyway, I'm aware that using "smut" for that—to mean "mature content"—is totally incorrect, as you might have gathered from my original post. It wasn't my idea to do so. The resulting confusion is bad. It is, however, the de facto current state of affairs on the site.
I am aware an overhaul has been in the works for some time. All above commentary is with regards to the site under the current state of the system (that is, as it has been for the past year or so at least?).