Ok, so Harry is the central character and I think it is therefore understandable that the majority of his friends and adventures are also with males. I know that when I was that age girls usually played with girls, and boys with boys. This was not a stereo-type, it was that boys had "boy germs" and girls I imagine had their own.
But you are describing crucial parts of the social institution of gender, which is the locus of sexism. This is part of a whole system of separate socialization of boys and girls in preparation for different roles as adults.
Hell, these are kids books! I dont see anything wrong with them, I think they are harmless and fun and that the portrayal of the girls/women in them is fine. Maybe I am wrong?
The fact that the books are for children is crucial. A huge part of childhood is the socialization of children into gender roles.
You need to get a wider scope view of what "sexism" is. In the most popular usage of the word, let's call this "folk sexism", it a used as a characteristic of individuals, individual actions or statements: "John is a sexist", "John did something really sexist by harassing Mary with porn pictures", "John said something sexist when he said women belong in the kitchen, barefoot, bearing children". There can also be an element of willfulness in the conception: people are sexist when they do certain things.
But in technical usage in feminist literature, definitions of "sexism" are about the way society is organized into (generally two) genders on the basis of their genitalia, the culture elaborates the difference beyond what is biologically given, and the result is the cultural hegemony of one of the genders. The members of each gender are socialized differently and inculcated with different values, but the values of one of the genders are in many ways dominant. Cultural values are not visible to the participants as such; either they are thought of as "natural" or "the way things are" or "normality", or they are not even noticed.
So, a basic tenet of feminism is that we live in a sexist society. This means that sexism is pervasive; we all have been socialized with sexist values since our birth. In this sense, as opposed to the folk sense, we are all sexist to some degree. Even if enlightened with regard to gender issues, a man still derives benefits from being a man, and women suffer corresponding disadvantages, through circumstances beyond of the personal control of either. And without even recognizing this situation.
Defending a work like Harry Potter from a plausible analysis as instantiating sexist cultural values by countering that it mirrors "reality", like you do, is not much of a defense, given that "reality" (in quotes because this "reality" has a strong, unrecognized cultural component) *is* organized in a sexist manner in the first place. Feminism has to oppose, as a matter of principle, *uncritical* portrayal of sexist social relations, on the grounds that such repetition entrenches this sort of relation in our culture and our minds.
The criticism of such portrayals, as the articles you link do, is part of the political project of feminism. It brings to conciousness how the "normal", "natural", "realistic" portrayals are gender biased.
BTW, if I were to explain the term "racism", I would do it pretty much the same way.