"a girl shouldn’t be thinking of those types of things" seems a bit sexist. The character is forcing a gender stereotype on the other. I don't think it's a big deal as it is, and that's why your teacher/professor didn't make it a big deal. Characters can be imperfect but a good author has to understand, acknowledge and even utilize that. They do not necessarily have to acknowledge it in text, but they must somehow. Just as it is difficult for an author to write a character smarter than themselves, it is also difficult to write one who is less discriminatory.