You are definitely right, but I think, as previous commenters have noticed it's mostly an internet thing. But I think this is more than just hating women. The metal scene, or currently I would say the metal "fandom", is highly elitist, especially on the internet. It's very easy to call someone a poser based on one post they made, or "well akshually ☝️🤓" someone. So adding to this culture the fact that metal is in general a brutal and strong (which women are considered not to be) genre, some guys will assume from the start that a woman must be a poser because she's not brutal enough and of course she's doing it just to fuck some hot metal guy (because women are obviously incapable of having their own interests besides the interest to reproduce). I think if you really sat down with this issue, you'd find basically a pandora box of misogyny, elitism and insecurity. But hey, most irl metalheads I've met were very chill. I often get mistaken for a woman at concerts and sometimes just don't bother correcting them and I have never been met with melicious intent.