Seconding periods and boobs as reasons why we stop sports.
Thirding it. Imagine two hypothetical children, a boy and a girl*:
The boy hits puberty. Suddenly he's stronger and faster. He may find it exhilarating to test his new limits. He's pumped full of testosterone, making him more aggressive and less risk averse. He may enjoy team sports more as a result. If he finds it difficult to manage his newfound anger and aggression, a sympathetic teacher may encourage him to channel it into physical activity. (Maybe one of our resident male contributors can also confirm whether or not heavy exercise and tiring oneself out can reduce the number of unwanted erections?)
The girl hits puberty. Suddenly she's getting attention from boys and grown men. She may feel self-conscious in her PE kit. Her breasts hurt when she runs and bounce when she jumps, and they hurt even more at certain times of the month. Her hips widen; she has to relearn efficient techniques for moving about. Her period causes her cramps and if she's unlucky, serious pain. It may be infrequent or unpredictable for the first few years. She may bleed through her PE clothes in front of boys or bullies. She may be allowed to skip PE class, but if she's forced to take part anyway, she'll probably start resenting her PE teachers. If she can't use tampons (out of TSS risk, sexual trauma, cultural reasons or an abnormally narrow vaginal passage) then her swimming career is over before it's started. Even then, there's no guarantee she won't bleed around a tampon or need to change one in the middle of an important race.
*Obviously, the boy and the girl are not real, nor reflective of a universal experience. Some boys hate sport and / or never develop the aggression required for certain games, like rugby. Some girls get luckier in their new physiques and are able to train their way to the Olympics. It's just meant as an illustration of how biology can encourage sport in one sex while discouraging it in the other.