Chihiro dressed himself with feminine clothes because he thinks he isn't manly enough. He never saw himself as a girl, he was just projecting how weak he is. When he saw Oowada, a buff dude that does everything in a exaggerated manly way, Chihiro went to him for training on how to be manly. Oowada got fooled by his appearance, thinking Chihiro was a woman and that he himself scored a chick that wanted to join him with his hobbies. When Chihiro told Oowada the truth weeks later and Oowada thought it was going to be a 'relationship upgrade', Oowada basically exploded with "NO HOMO NO HOMO NO HOMO NO HOMO" at the reveal and killed Chihiro. Basically, Chihiro trapped Oowada, even if it was an accident and led to an unfortunate result.