@ze  
Eh, it’s more like Superman has no special defense against magic. And even then, only high-end magic users like Discord or Twilight at her most powerful would cause problems for him: a magical punch from Rainbow Dash wouldn’t hurt much more than a normal punch from a normal human(that is to say, not at all), if that makes sense. Magic’s not an Achille’s heel like kryptonite or red sun radiation.  
Also, even if Rainbow’s magic punch could hurt him, Supes would be fast enough to react to Rainbow’s cheap shot, seeing as how Supes is many, many, many times faster than Rainbow.

Actually, Rainbow Dash is a magical talking pony, and we all know Superman is weak to magic. Maybe that could be the weak explanation behind this.

Is he can take a bullet to the eyeball literally without blinking, I’m pretty sure that crunch sound is her hoof shattering.

@platypusmac  
Well yeah. They’re golf ball-sized wrecking balls.

Actually, someone tried to kick Superman in the nuts in the comic. He hurt his own foot doing so.

@sargesprinkles  
Dash must have a kryptonite or magic horse shoe on.

why would his testicles not be as strong as the rest of him?