Seems to me that alot of guys just assume they are sposed to like metal, gals on the other hand seem to like it because they "aren't supposed to". Both genders exhibit "wanna be's", normally given away by their "main stream" or "big act" t shirts and their overwhelming urge to "fit in" to what they assume to be a stereotypical group (metal heads). When the truth is metal crosses all spectrums of types of people apart from genders. Social class, goegraphical location, age, appearance, occupation and so forth mean nothing. There is metal everywhere of all kinds for everyone to enjoy, and that is the true beauty of metal.
With that being said, women being more emotional by nature are likely more moved or gripped by a given piece of metal. They will listen to it alone, nobody would suspect alot of them to be "metal heads", and they listen for themselves. Not to say there isn't lil sluts running around in bathory or darkthrone shirts who never dwelled past the mainstream act of the top listed acts in their "preferred" sub genre or metal
Males on the other hand, more egotistical, and boisterous by nature. Seem to hear it from a friend, buy the shirt, hang out with other "metalheads" and go on liking it cause it revs them up and it's what buddy likes, and it makes you "tuff" compared to the white kids liking rap and top 40 crap. Alot I feel dont "feel" music as others may, they are just self branding them sorry selves. Women being more "emotionally aware" I feel gives them an upper hand to understanding any music not just metal, better than us lowly simple males...
While this sector of music is male dominated, as I mentioned earlier metal is about breaking stereotypes, the norm, and unadherence to social pressures, gender or any other way of classifying a human means nothing. All kinds of people all over love all kinds of metal, and it's a beautiful thing.