Comment by Hyakkimaru on The Regressed Mercenary’s Machinations - Chapter 34

@Hyakkimaru because shonen?
Thats like asking why shojou has comically large eyes

@Ianes Grécia Shonen? You're overestimating things. The majority of readers are male, and most of them just want to see bo***, age doesn't matter—it's as simple as that.

@Hyakkimaru so the oversized male muscles don't bother you .......I wonder why

@inhuman  shonen is literally 'category for young male reader'

@Ianes Grécia We're in 2025 — classifying content by age is about as useful as those checkboxes asking if you're over 18 when you enter an adult website. What really matters is whether there are suggestive scenes or explicit content being shown. Even seinen—which is supposed to be targeted at adults—gets censored, because everyone knows that anyone can access it anyway.
Besides, gender classification is pretty much just for research purposes at this point. Honestly, by the 21st century, gender doesn’t matter that much when it comes to enjoying a story. There are plenty of girls who enjoy shonen stories, and maybe not as many, but still a good number of boys who watch or read shoujo too.

@inhuman its because of westen believe of "classifying content by age or gender is outdated" that its entretainment media is failing while the east that still holds it are triving. You can believe what you want, but these categories are not only still very useful for the writers, but for the readers as well.

@Ianes Grécia What I believe is easy to understand : as of today where a child can have access to adult content from their smartphone, the classification does exist only to legally protect those who upload those adult content (and also for the purpose of when reader is searching for specific content as I stated earlier).

Bro started a fight and dibed

@inhuman this justification is also NOT EVEN RELATED the reason the categories exist in first place.
The categories of manga are not there for 'legally protect those who upload those adult content' these are not R-rates.
These categories are about the kind of TASTE.THE AIMED AUDIENCE MAY PREFER. That is what AIM FOR AN AUDIENCE means. 
That is also why shonen manga still has content like gore and po** even when aimed for teenage.
That is also why each category has some kind of trend feature like shoujo eyes. Its because said audience, when looking for that category, EXPECT TO FIND THAT KIND OF CONTENT, even if they are not part of that specific audience.
That is what I meant in 'its helpful for the writers and readers'

@inhuman So yeah. Here is the basics:
If looking for super-hero fights/ journey of hero, look for shonen
If looking for magical girls/school romance, look for shoujo
If looking for brooding protagonists, look for seinei
If looking for slice life drama, look for josei
These are VERY LARGE strokes of what each category represents and are not exclusive to one another, but when a reader LOOKS FOR ONE OF THE CATEROGIES they expect to find at least the most basic of each of said features.