Honestly I think it's more about a lack of a reason for him being 16. Most decent BL, even ones with a lot of smut, that feature sub-18 year olds have a reason (like the characters being in high school, or their age being part of the naivety of youth). But there just straight up isn't a reason why these characters aren't a more reasonable age, other than the top being a kid and looking like a kid, specifically as fetish content. And that just rubs a lot of people the wrong way (reasonably so).
If it had at least pulled a "he's a 20 year old shota" some people would have looked past the "looks like a child" bit. But knowing even that isn't an option is a deal breaker for a lot of people, so it's nice to have it in the comments.
I acknowledge that but i also see alot of ppl nowadays be uncomfortable no matter the setting. They see the "16" in a smut bl and go rampant then proceed to praise popular highschool yaois (and in popular yaois in general) when theyre in the same boat. Pretty sure they only reason why these ppl are freaking out over this bl specifically bc it was brave enough to verbally mention their age. Kinda hypocritical

Okay i get thats its bad cuz hes 16 but i thought we were used to dismissing this? Ig yall would be surprised how much yaoi (especially ones set in school) feature characters under 18