Why are there fewer women in heavy music...sexism, sexism, sexism. Why do people ask such silly questions?

But anyways, if you look in the depths of death and black metal you see some pretty good female musicians.
One of the best bass players in death metal, imo, is Jo Bench from Bolt Thrower. She's been with them since the beginning (80's) and is a monster on bass. And unlike all those "gothic symphonic melodic ladeeda" bands, she doesn't cake herself with makeup and try to be a diva.
There is also Rachel Heyzer who used to do vocs for Sinister, Infinited Hate and a few other death metal bands, who has really impressive grows. She unfortunately hasn't done much in the last few years, though.
My favourite is probably Masha Arhipova from Arkona (Russian folk metal band). Her growls are ->-bleeped-<-ing amazing and she is totally bad ass on stage.
and more Arkona

:
Darkened Nocturn Slaughtercult's vocalist is also female: