If there's a girl with potential in a club, she'll start training with the boys(assuming the club has brains). The reason is that in order to progress, you need to train with people of a higher standard than yourself. It's precisely because it will be dominated by boys that the girls prosper. You're not supposed to dominate in your teens, that should be happening when you're 18-20 when you actually start competing.
And come on: you said sports, not sillyness like rugby. That activity is just a substitute for people who are too afraid to get out of the closet, who choose a socially acceptable way of having intimate relations with fellow males.
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