From my perspective, it was like "bullying inception". I felt ashamed that I wasn't enough of a boy and also not seen as enough of a girl that I was throwing perfectly within societies expectations. So the insult hit me twice :-(I think it's just about physical strength, it was very common to hear that when I grew up. "Throw like a girl" and "fight like a girl". Boys were supposed to be strong and hard, with more muscle than the girls had. So not really about being bad at throwing, just less strong.