The article says that the mother of the girl filed the complaint. It doesn't indicate whether anyone has filed one in the other direction; perhaps the guardians of the boy find the idea of prosecuting this behavior repugnant. (And I think it's common for the authorities to choose not to pursue consensual underage cases in the absence of a complaint.)

The complaint made by the girl's mother merely made the state aware of the situation. The state is prosecuting. It is free, and some would say obliged, to prosecute the girl as well, since it has the same reason to she has committed a crime.

Sexism isn't that hard to explain.

No, I know that's what it is. I just want to hear the justification from the legal system how it's anything but that (assuming there even is one).