Way back in the old days, there was a female tag, but it was purged for reasons documented in topic #3586.
While there are some searches that would probably be slightly less cumbersome with this tag, the point remains that it isn't strictly necessary. For many of us, nonamethanks's recommendation of searching for -male_focus provides results that, while not perfect, are good enough. Yes, you will have to deal with false positives, or append -1boy -multiple_boys to your search if you really, really don't want to see male characters, but this alone culls the number of unwanted posts by a huge margin.
In hindsight, was removing the female tag a bad idea? Maybe. But it's gone now, and at this point, recreating it would require a monumental effort. It's been gone for over a decade, yet Danbooru has done just fine without it. I think I speak for most of the contributors here when I say that I'm not feeling very enthusiastic about populating a marginally useful tag with literally millions of posts. It's just not worth the trouble, and we have bigger fish to fry.