Perhaps they consider it "unsophisticated" to embrace warlike feelings and masculinity in music. Maybe they associate aggression in music with blue collar idiots.
Yes and yes, mostly the latter, because most metal fans are alcohol-addled teenagers. So were many of the great metal composers.
So what albums/songs would you introduce a classical listener to metal with? And how can we destroy the mental barriers between "intelligent music" and "aggressive music?"
I wouldn't appease like that, though I did in 9th grade: "But Mom, 'Master of Puppets' is like 9 minutes long, and they listened to Beethoven!'
I've found that musicians (though non-Metal) will recognize that something is going on in the great works, and maybe even suspend their aversion to the distortion and growls/screeches--it's at least interesting, to those who care about composition.
You might destroy the mental barriers between Intelligent and Aggressive music when metalheads cut their hair, kill the juvenile delinquency, chill on the drug abuse, go to college, and become normal. But then, would the great works have been made under those conditions? DANG IT. This shit was never built to make sense to the average indie-popper. You can't convince fuckers to like it, only highlight the awesome characteristics that they've overlooked.
Still, start from the canon, and discern if the listener would vibe more with composition/meaning, technique, atmosphere/production, vocals, lyrics, whatever.