I think it's also part of that "I'm totally heterosexual!!!" trope in BL. I don't think Shion even registers the fact that a guy could be a potential romantic/sexual partner (other than Rui) because he's lived his whole life as a straight man. I don't think he does it on purpose. But I totally see your point
i just think he's confused rui can't just expect shion to fall in love with him over night that's not how it words simple
No way Rui just literally forced himself on Shion cuz he misunderstood him having a conversation and smiling with another and you're saying hes the sad one?? Shion sees him with girls and gets jealous of how easy it is for him but also there is the fact hes unsure of his feelings about all this. It's all new to him just cuz he turned into a girl and now hes feeling things for his friend he never expected. Rui says he doesnt care whether hes boy or girl shion but that isnt the same as how he feels for him emotionally and the everytime he gets he sleeps with Shion but sleeping together isnt a sign of true romance when he still flirts outwardly. They should both talk seriously.
I honestly understand that shion is confused but.. He's treating rui like a fucking piece of garbage in the process of being confused he's literally playing with ruis feelings. in my opinion i find it unfair that shion gets hella jealous & mad when rui talks to girls & goes out with them but then he ends up talking & going out with other guys.. like how come you can get jealous but do the same thing. in my opinion shion just kinda uses rui to his wanting and thats just my opinion on it.