I was wondering if there was a prerequisite to the guy's catching the gender-swapping disease. So far it seems that if you are a guy and you develop/have feelings for another guy then you will develop the disease.
When the MC went to tutor his cousin (a boy) the cousin develops the gender swap disease in front of the MC. Based on that I am thinking that the cousin had some feelings for the MC as evidenced by his enthusiasm while waiting for the MC to arrive. The cousin even though he was 12 wanted couldn't wait to play games with the MC, didn't want the MC to go home, and wanted to take a bath with the MC.