https://en.wikipedia.org/wiki/List_of_The_Simpsons_cast_memb...
In no way does it look like the cast is "mostly male", or that the women are cast into single roles.
The Simpson's were kind of famous for Bart being voiced by a woman, meaning the Simpson family has 3:1 Female:Male voice actor ratio.
They're not for everyone, and the voices are still mechanical, but I've become a huge fan of English Vocaloid, especially Gumi.
Take Tara Strong. Sure, lots of her voices sound a little similar (you might hear Timmy Turner, then hear the girl in Batman, and think "oh yeah, they do sound similar" once you knew they were the same voice). But she can definitely "transform" her voice.
The male voice actors on The Simpsons have incredibly transformative voices, which I think comes from lots of practice in the industry. The first time I saw a chart of all the voices for each character they play, I couldn't believe it. After paying very VERY close attention, I can kind of hear Mr. Burns in Moe, but just barely.
Edit: he also did a made-for-YouTube miniseries wherein he re-enacts actual transcripts from the Nixon White House tapes: https://www.youtube.com/watch?v=f9HtoWea72A
Which is just how it goes. It's unfortunate but you can't strike a 50/50 balance. Even for side-characters written as female in the script, they're going to become male during production because you don't have female voice actresses who can make a "different" voice; it would just sound like Lisa or Marge. Really only Bart's voice actress can do it (she does Nelson, too, who sounds very different).