Why is making the generalization of "boys usually being more into sports than girls" an unfair generalization?

Because it is incorrect. If it was a correct generalization you wouldn't be getting much backlash. This generalization is born from centuries of people believing that a woman's place is "in the kitchen." It's not because it is accurate. Did you miss the comment I made about studies confirming that girls participate in sports at the same percentage that boys do? That was the backbone of my comment.

Yes, there are some biological differences in the brain of males and females, however interest in sports is categorically, undeniably, NOT one of them.

Your red pill rant at the end kind of makes me sad, and I think perhaps you are quite a bit more biased than you think you are. It's not just "radical libtards" that think girls are into sports, it's every person who has ever actually met and listened to what a girl has to say.