Sure, I guess that's an option for youth sports in the prepubescent age groups. As a practical matter most youth sports leagues and schools aren't going to hassle with sex screening tests for little kids.

But once puberty hits everything changes. My teenage daughter played travel club volleyball on a pretty good team, and during practice they would occasionally run drills with the boys team. Even at that age the difference in hitting power and vertical was enormous, and those differences only grow larger with age. Men and women are literally playing different games. Beyond just fairness, forcing girls to compete against biological males becomes a safety risk due to concussions from taking a ball to the head.