IDK how old you are, but saying that you've never seen any other fandoms act this way makes me inclined to believe you have limited life experience.
The Marvel fandom has done all sorts of crazy shit and has had degenerate behavior.
TV shows like GoT have had degenerate subsections of the fanbase.
The star wars fandom and the LOTR fandom have also had the same sort of degenerate subsections of their fanbase.
Heck, even game fandoms have had degenerate sections of their fanbases like this game or older games like DnD.
There are plenty of examples of the fanbases of other types of media exhibiting degeneracy. Degeneracy in general has permeated all forms of media across the world. However, just like there are decent shows and movies, there are very decent and even incredible anime. A few that come to mind are 86, Vinland Saga, Death Note, Violet Evergarden.
I'm not suggesting you watch anime, but I do recommend not judging someone just because they do.