You're just not talking to the right women. If you see a woman wearing a metal shirt? Go for it in a non-obnoxious way.
It has been reported that Amon Amarth shows have lots of women there... and women tend to attend folk metal shows in larger numbers.
Or you can go to a shudder Dragonforce show.... or Ozzfest. Something mainstream.
I happen to be a die-hard metal fan who happens to be female. I found it difficult to find a boyfriend who likes metal, but I finally did. He's great (even if he does like cheesy power metal and calls everything with harsh vocals 'black metal').
I'm not really sure I could date a non-metal person (shallow as that sounds) because it's a lot of who I am at this juncture. I need someone who won't tell me to 'turn that crap down' and will understand (at least a little) this 'metal mentality'.
Part of it might also be that, in many cultures, it's completely unacceptable for women to express anger. Whereas men are shipped off to prisons for expressing anger, women are sent to mental hospitals. It's not considered a normal state for a woman to be angry. It's considered a lot more 'normal' for men.