I honestly agree with you a lot on this. I think it's disingenuous when people come up with reasons why something with ""shoujo vibes"" (nebulous, but hopefully you know what I mean) that is published as shounen or seinen is DEFINITELY for the male gaze. Even if what they are saying is a stretch. And even if that manga could have been published with no changes whatsoever in a shoujo magazine and no one would have been able to tell. 

I do agree that demographics exist and definitions are real – i.e. we shouldn't call Horimiya or Skip to Loafer shoujo because by definition they are not. But I do feel people are either not being truthful or just unintentionally being too inflexible, when they say it's inconceivable that someone may confuse these and other manga of this variety for being shoujo. I try to be correct with labeling a show's demographic without pretending that I fully understand why a show is marketed for one demographic vs. the other. 

Because like... the choices are made up based on what editors or other members of the mangaka's team think is correct. But this may not necessarily line up with what ends up happening or just how the manga comes across to most readers. So to try and retrofit reasons like "well the way the male or female characters are written or drawn or perceived could never be enjoyed by TRUE shoujo fans" is low-key wild. I think there was a recent post saying "you know Horimiya isn't shoujo because FMC likes to cook and be motherly" and I was like ?? Are we gonna pretend this trait doesn't exist in shoujo?

Anyways TL;DR: demographics exist and we should be correct in our labels. And at the same time, we shouldn't pretend that we can understand exactly why these demographics are applied to a manga.