This seems to be a point of contention that's cropped up a lot lately. I just think the best solution is the one that lets the most people get what they want while not harming anyone, or the fewest people possible if someone has to be harmed.
While I agree that enforcing the policy will be difficult, and that most girls won't want to play hockey with boys, and that even having to have a policy in the first place is a pain, none of those are good reasons for keeping the girl in the article from doing what she wants.