I didn't find any actual statistics but anecdotally the audience is primarily male by a good margin. That appears to simply be a function of the music. Heavy Metal fans are primarily male. If you go to a Howard Carpendale concert you will find that almost all guests are female.
Wacken is considered to be very safe and friendly. The local police considers it "safe". Incidents are rare and the main reason for injuries are accidents with camping stoves. This is a very large festival with lots of beer, mud, crowd surfing, mosh pits, and loud music. Everyone needs to decide for themselves whether that's their idea of fun or not. While incidents of sexual aggression are not unheard of, they are exceedingly rare and less likely to occur than in your average bar on a Saturday night (which should not the case, but unfortunately is).
Most people in my circle of friends are bothered by the fact that the vast majority of visitors are men.
Why? Action movies are more often watched by men and rom-coms are primarily watched by women. This is exactly the same thing: there just happens to be a gender bias in media preferences both ways. What's wrong with that?
Postscript: A friend of mine was working security at a very large concert venue in Germany. According to him, the most aggressive fans he had to deal with were indeed middle aged women at a Howard Carpendale concert. Metal fans were comparatively well behaved.