The most stupid thing about it is that from a physiological perspective, women should be far better at audio engineering than men. Women are (typically) less interested/obsessive over small details and more about the bigger picture, they're better at hearing things as a whole rather than focusing on one aspect, and they tend to hear things more evenly rather than just using their dominant ear.
Honestly, I think it does come down to sexism of a sort. Not in the "women can't do it" sense, but in the sense that most guys in metal bands are men. When you're in the studio, you want to relax and be comfortable etc. - and most men (whether they realise it or not) still act differently around women, especially women they don't know. The language is less blue, the jokes aren't as filthy. Of course everyone will claim that's not the case, but it really is.
It works the same the other way around too. I've been in 3 bands with female singers; one proggy, one metalcore and one grindcore. As the music got heavier, the audio engineers we worked with got less comfortable with the woman. The guy that recorded the grind band could barely look at the singer (and she was really worth looking at), and hardly said a word when she was around. As soon as she was in the vocal booth, he started joking around - then stopped as soon as she came out. With the prog band, the guy was used to recording pop and punk bands, and he was the same with the chick as with the rest of us.
Steve