The mass of people who know the band call them some variation on DM, thats just a fact, and as we know, whatever the masses say is right. As someone said somewhere else, the Rasmus are considered Goth by kids of today, and they are right, because its those people that decide these pathetic decisions on genre and what is cool and what words mean. Like it or not thats the way it is and always will be. The Rasmus are gothic, opeth is DM of some kind, Kerrang! is a really good magazine.
There is no point debating it. People argue over genres all the time, and even if everyone on this forum agreed that Opeth is no where near DM they would still be called it by the masses. So whats the point in arguing over something so pointless?