But it's also more complicated than I thought. When Rowan's lifelong male pals play happily with her at home and ignore her in the playground next day, she feels hurt, mystified and confused. I feel the hurt on her behalf. How on earth to explain the Great Injustices of the World for Women in one satisfactory sentence? What I never realised was the agony of dilemma that boys experience - nor the invisible hierarchy which enforces it.
Since P1, Rowan's male friends, like Joey and Pete, have used all their childish imagination to juggle the demands of conflicting loyalties, tact and peer group status as she tries to join in their games and conversations. Rather than say something unkind, they pretend not to hear or run away, or they say nothing as the Smith brigade shouts abuse about useless girls. She wonders why they don't stick up for her; they feel guilty, yet lose face for not joining in the rude remarks.
I used to assume all boys began feeling exclusive early in life, once they got into groups. Instead, the interesting thing is that all Rowan's friends complain in private about individual boys making the rules. Rob and Ian won't let them talk to girls, ridicule them or beat them up if they do, and often they're the same boys who bully them about other things.
Of course boys and girls segregate themselves naturally for a while, and girls can become just as scornful about boys. Nobody is asking the impossible - but a dividing line between the normal pairings-off and prejudiced, sexist abuse has to be worth making by parents and teachers. Primary school seems the place to start.
I think the experience of kind Joey and Pete, who have already made their choice out of fear and will go on doing so, actually gives grounds for optimism. Because the problem isn't universal, it's identifiable: with support they would act a bit differently, and feel a lot happier.
Smith's different forms of aggression are all inter-connected. How many teachers think of including everyday sexism in discussions with primary kids about bullying in school, and how to tackle it? When they talk to boy bullies, how often do they think of exploring their attitudes to girls, and where these attitudes have been learned?
But I should practise what I preach. Full of grand ideas about exploring strategies of resistance with shy Pete, and explaining in simple terms the dynamics of male bonding with Rowan, I can only tell her lamely at bathtime: "Boys are a bit funny, you see, they worry about daft things."
This is followed by shameful betrayal of my principles: "It's really hard for Joey with that bully around - might be easier if we went and played with him at his house." Ah! Females putting it right for males, again, as ever: must do better, as you teachers would write on our reports.