Actually, with the exception of Nidorans, _none_ of the Pokemon, in the 
 game or show, has a discernable (or mentioned) gender.  Pikschu can be 
 male or female, depending on your choice.  Most fanfic assume Pikachu's 
 male, but at least one, Pikachu Girl-Type, has Pikachu as a female.  

 I would assume that Pikachu is a girl, because she has a voice actress. But
as far as I'm aware, the TV series has never called Pikachu a girl or a boy.
There was this one episode where a Magnamite (Who sounded like a guy) had a
crush on Pikachu. So, I would most probably state that Pikachu is a she, and
not a he. 

This is, of course, assuming that the Pokemon are not asexual. 

  ___________
 / Here lies:\  Save the Whales -> Harpoon a Honda
|   Andrew    | 
|             | The light at the end of the tunnel may be an 
|   I think   | oncoming dragon.
| he's hungry | 
|             | What UNIVERSE is this, please?