Ponyboy is canonically fragile. He gets beaten up, he passes out in burning churches, and he suffers from PTSD. Readers who write him in a "bottom" or submissive role aren't emasculating him; they are protecting him. When a girl rides Ponyboy "top," she is controlling the environment. She is ensuring he doesn't have to perform aggressive masculinity. For many readers, this is the ultimate form of intimacy: a girl taking the lead so the boy can finally rest.