The scene that really hits it home for me that serirei is micro tiny bit intended is the part where Reigen asks about Serizawa's previous crushes. Because it is like textbook trying to see if someones is also gay. (PS Im gonna read wayyy to far into this because I love this ship)
Chapter One: Internalized homphobia
"But there must of been some girl you liked" "at that age, even if you laugh it off,that's a bit troubling" (paraphrasing btw) I know its not cut and dry 'I hate that I'm gay' butttt like here me out. This reads to me as him having some outdated views (on himself and probably other adults) on gay being a phase. Like "yeah as a kid we all had crushes on boys but we all had to grow out of it".
While I do think Reigen is in definitely supportive of queerness, I think he has a hard time accepting it fully with his own insecurity. Thus it being troubling that Serizawa hasn't grown out of this bad habit, like Reigen. Internalized homophobia is really hard to explain in words so I really hope that you understand what Im saying.
Chapter Two: Pushing back on that homophobia
"Oh wait are you serious?" He says with a smirk kinda thing. I take this in three ways; backtracking from what he's saying because of the realization that Serizawa was a Hikikomori and also backtracking that its "troubling" because if he is gay than its aye okay (probably trying to get over his views or some).
The smile implies that he's rather dense as shit (total possibility) or he's like wait if Serizawas gay than maybe I can be gay
I don't think its out of 'yay new bang buddy!' I think its more that another gay person makes him probably feel a lot more secure in himself.
Chapter three: The gaydar method
These days its harder to guess if someones gay or European so as someone who somewhat relates to Reigen, I feel our gaydar process is similar.
-Make a gay joke a see the reaction (often times leaning negative because its safer to self deprecate than be like haha what if we held hands and watched a show together haha)
-If the response reads okay-with-gay, push deeper and figure out if their and ally or straight up not straight
I know its just two steps and not that cut and dry but idk I feel like I may be onto something.
Even if serirei isn't cannon Reigen is definitely gay and that will always be cannon to me
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