I guess I've almost exclusivly dated girls who are into metal to various degrees. I don't really think it is that important though, but it is natural when you used to hang at metal clubs, have metal friends, etc. Long hair usually attracts that crowd too.
My wife now shares some of my tastes and tolerates pretty much everything (except Tony Wakeford). She does like Kent though, the worst band in the world which is abit awkward.
Most metal girls aren't really into the same stuff as me anyway, I mean I don't think I have ever met a girl who likes albums like (as in a big fan of not just thinking it is good if I put it on) Awaken the Guardian, Battle Cry, Dark Quarterer, will of the Gods is Great Power etc. It is either 70s inspired stoner/doom/retro occult rock shit or it is full blown black/death style. Sword and Sorcery doesn't really attract girls.