I speculate that those girls who do throw like a girl still throw like a toddler, whereas boys (and girls who play softball regularly) grow out of it and use a more efficient motion of the arm.
But I'd never heard of kicking like a girl. Apparently it's real. Some researchers hooked up a couple of dozen male and female college soccer players with electrodes, reflective markers, and video cameras everywhere, and they found that
They found that male players activate the hip flexors (inside of the hip) in their kicking leg and the hip abductors (outside of the hip) in their supporting leg more than females.
In the kicking leg, men generated almost four times as much hip flexor activation as females (123 percent in males compared to 34 percent in females).
In the supporting leg, males generated more than twice as much gluteus medius activation (124 percent in males compared with 55 percent in females) and vastus medialis activation (139 percent in males compared with 69 percent in females).By itself, this is just a moderately interesting difference between the sexes, but it's also true that women suffer many more injuries to the anterior cruciate ligament (ACL) than men do. The authors suggest that the greater use of hip flexors and abductors by men may protect them against injury to the ACL.
It seems to me, not being an athlete myself nor associated with any, that this should be a coachable thing. If girls are using different muscles to kick with than boys, then they are somehow swinging their legs differently, and that should be teachable.