Why do you assume they're uncomfortable with the given context? Not attacking. Just curious because I only read one comment and they even tagged it as baseless. They could simply just be fabricating for fun.
Let me give you an analogy. You know YuruYuri? It's an immensely popular series with zillions of followers. The girls in that manga are in 1st or 2d year middle school, they are barely older than Mai. Well, get a load of this: nobody, and I mean nobody, has ever ever speculated that they are actually a bunch of 25-year-olds masquerading as seventh and eighth graders. The canon says that they are kids in middle school, so everyone thinks that's exactly what they are. Claiming otherwise wouldn't seem fun, just weird.
But then why when it comes to BCO, even tho the canon is as strong as in YuruYuri, so many readers express such a strong reject of textual fact—coming up with explanations for what doesn't need to be explained and positing that Mai has to be an adult woman, even tho this idea creates a shitload of massive complications and is way more implausible, far-fetched and problematic than the simple canon?
The most seeming answer is: because they are uncomfortable with the given context. As for why they feel this way, well, I think I can guess... but that would be a speculation of mine so I won't say it.