Yeah, to be honest, I had a problem with that even in the comic books after the 1986 post-Crisis reboot, since prior to that, Superman's invulnerability made it impossible for his hair -- including any facial hair -- and nails to grow because hair and nails are nothing more than modified dead skin cells, and invulnerable skin cells can't die or be shed. Changing that rule post-Crisis so that he COULD grow stubble was supposed to make him a little more human, but to me, it diminished one of the things that set him apart from other superheroes.