While it's true that girls and boys are not that much different physically pre puberty, once boys hit puberty and testosterone starts doing it's thing, it's pretty much over.

At the first school I taught at, a girl went out for football. She started with hell week and could keep up with the conditioning, but when they got into pads, things changed pretty quickly. The first time she tried to take a hit she absolutely got lit up. She got hurt that first day and was relegated to JV. She did try to stay with it for a week or two, but the coaches had to tell the boys to kind of take it easy, because she was going to get seriously hurt. Even when they did, she still got hurt. She ended up as a trainer's assistant.

True story