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im not exactly sure what u meant by toxic, is it on the reader or the story and the character itself? if it's on the reader then, often became toxic because ppl tend to fetishize gay men--hot gay men having sex.
but if it's the story and character, personally i think the toxic part is often on the top. mainly because they "rape" but ended up romanticizing the rape itself so it "looked" okay and sometimes they also do manipulative sex.. and don't even get me started on drunk sex, also how they suddenly become deaf when having sexual intercourse. hmm idk what else though
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A lot of BL mangas have rape like the user @kay talked about, but it's not just that. There's a lot of stuff which makes the gay couple in the manga hetero. For example,
1. the bottom looking like a girl with a dick and without any boobs
2. the bottom being called the top's 'wife', as opposed to being called his husband as the bottom also identifies as a man.
3. Bottoms (not including omegaverse) having a reaaally sensitive ass which gets wet for some reason. I have never met anyone with a self lubricating ass irl, tell me if you have.
In conclusion, it's just BL manga showing incorrect representation (it's not like I consider it as gay representation) of gay guys irl AND 14 year old girls ACTUALLY believing them and implementing that weird shit to real life gay couples.
These are a few that are coming up in my mind. If I am incorrect anywhere, please correct me and I apologise for any grammatical errors.
Heyy am curious about what the others saying about the toxic of a bl manga manhwa or manhua pls educate me