Bro, it’s the first man for him. He’s just overreacting/overthinking it. You’re right. It’s not that big of a deal. They are good friends overall and he’ll most likely get over it.
I saw it as more of Shion discovering that part of Rui despite them being best friends for such a long time. Is he overthinking it? Yes, but only because he's (though unaware atm) in love with him.
Jesus if he’s fucked another man it’s exactly the same as him fucking other women! Get over yourself you little bitch!!!!
sorry I’m just getting sick of this trope