There's nothing psychological about this. After this incident, if this bitchboy even considers to give the girl a chance, then I wish him a very pleasant dick chopping.
Besides, there is objectively zero reason to pick the girl in this scenario. I am not even gay. But you have a boy who looks exactly like a girl and then you have the girl who just looks like a girl. Choosing the boy is like getting a convertible, he always has the option to go to the snipper snapper. But it doesn't work the other way around. And how many millions of blonde girls exist in this world? Now how many blonde boys exist in this world that looks 99% like a girl? Exactly my point. It's like a shiny Pokemon.
The only vanity of the girl was the boobs, pussy (both easily replicable), and her kindness which she just lost. Can we just end this manga by the next chapter with Rikimaru loosening Kyou's ass in front of the sister?