I'm not sure if there are more male characters to begin with (and I'm not even going to try to count, haha), but it's fair to say that characters that tend to return for more than 10 episodes are male (i.e. our main leads, Bobby, and Castiel -- I can't think of any other non-dead characters that have appeared in as many episodes as Bobby and Castiel). Females are not always well-written on SPN, but in terms of sheer number, they're not underrepresented.

Some people mentioned that fans' reactions to Castiel would be very different if he were a female, implying that Castiel would not be nearly as popular -- which is an interesting point, but I haven't really thought it through yet.