I think it has more to do with socialization of sex roles. Metal is "strong", "hard", "loud", even hostile and violent, all which a woman is supposed to be the opposite of. As the years passed, more metal subgenres evolved, women liberated more and got in to metal as well..the same goes for rock and punk music.. i dont think female metalheads are that rare in Europe, sure men prevail but there isnt a huge gap anymore. But i do believe that there are less women in thrash/ heavy, oldschool metal than in symphonic, gothic, power, doom, even black..mostly because these genres are more modern. But rock/heavy metal sure was chauvinistic in the past, females were accessories and sex objects just as much as they are now in rap/pop. Still, some percieve metal girls more like groupies than actual metalheads.