In some cases, it could be an accident. For example, if they were in a chaotic situation like a crowded concert or a rowdy sports event. The girl might be moving around and accidentally kick the guy in the nuts. However, in most stories, it's usually due to some form of provocation from the guy that leads to the girl's reaction.
There was this girl and a guy who were in the same sports team. During a practice session, the guy accidentally pushed the girl really hard while trying to get the ball. Annoyed, the girl instinctively kicked him in the nuts. The whole team was shocked and the guy was on the ground for a while moaning in pain.
The boy would be in a great deal of pain. It could be so intense that he might not be able to stand up straight away.
Well, that's not a very nice situation. Maybe the girl was really angry at the boy for some specific reason. For example, the boy might have said some mean words to her, and in a fit of rage, she lashed out and kicked him in the nuts. This kind of action is inappropriate though, as it can cause a lot of pain to the boy.
One reason could be for a performance, like in a play or a movie. Acting often requires actors to step into different genders to bring a story to life. Another reason might be for a prank or a dare, just for fun among friends. Sometimes, it could also be for a social experiment, to see how people react to someone breaking gender norms.
One reason could be for a performance like in a play or a musical. For example, in some classic plays, there are female roles that are so iconic and require a certain acting skill, so a guy might dress as a girl to bring that character to life. Another reason could be for a prank or a joke among friends. They might want to see how the guy looks like as a girl and have some laughs.
He might be looking at the other girl simply because she is in his line of sight while he is lost in his own thoughts. It doesn't necessarily mean he has any particular interest in her. For example, he could be waiting for someone in a public place and his eyes just happen to land on her while his mind is wandering.
One reason could be self - defense. If the guy was harassing or threatening the girl, she might fight back to protect herself.
Well, it's really not a common scenario. Gay men are typically not sexually attracted to women. If it occurs, it could potentially be a result of experimentation. The girl might be curious about sex with someone she trusts, like her gay friend, thinking it would be a 'safe' or different experience. And the guy might be going along with it for various reasons like wanting to please his friend or being in a very unusual state of mind, but this is all very abnormal and can lead to many unforeseen consequences.
Perhaps in the context of the story, there is some sort of mental or emotional instability involved. The woman could be suffering from some psychological issues that cause her to act out violently without proper control. This would then be a story about the boy's recovery and the woman's journey towards getting the help she needs.
Well, without having read this particular fanfic, it's hard to say precisely. It could be part of a more complex plot within the fan - made universe. Maybe there was some sort of conflict or misunderstanding between Cho and Harry that led to this extreme action. It could also be a very strange and out - of - character interpretation by the fanfic author.