i haven't read the manga. I have seen both seasons of the anime? Is yuri like extra prevalent 11+ volumes in?

I know that one of the major side characters (who is i presume responsible for the Lolicon tag as well.) has a female slave. But isn't the manga mainly about our shota hero and the not so big breasted red head heroine?

I thought Yuri shouldn't be a thing unless its like a central theme?

Having a series with dozen of lesbian sex and rape scenes and a cast of predominantly bisexual girls is the definition of a "central theme". Having a strictly lesbian deuteragonist doubles down on that which is the case here and is the author´s fetish. Both of them. See their other crap.
Tag what you see, it is as easy as that. ANN further has "theme" tags that are voted on by the community. The Yuri tag is equally present for both seasons. Again, tag what you see. Is what you see a recurring theme? That´s your leitmotiv. Is there a genre for it? Congrats, you found one the work´s genres.

Why this isn´t Shotacon. Thirteen makes the protagonist a teenager. It´s still child pornography and lolicon due to sex/rape scenes with eleven-year-olds. Not a teen. Thus Lolicon. Counting and logical deduction is all genres are.

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

Damn dude. Do you get this question alot?

Well thanks for explaining. Might check out author's other works.