What if Sherman was... formerly female?
Hold on, hold on hear me out, here (the grumpy old Mica miner says, over his helping of Infinite Bread and cup of coffee):
-- For one, and this is pretty random, but Narrator Sarah almost always voices Sherman, except in the first few episodes where he appears.
-- Tzila says to Lark that she "doesn't exactly have a Mom," but she very notably did NOT say "I don't have a Mom" or "my Mom is dead." She speaks with a lot of hesitation, as if explaining this is difficult and fraught. We don't know much about the acceptance of non-hetero sexualities in the world of Midst, but her difficulty is... notable.
-- When Hieronymous sees Tzila as a teenager, he says, "She looks like Sherman," and Saskia replies, "but she's got your nose." This implies that there is some actual genetic mingling going on. The world of Midst is weird, so maybe there's a way for this to happen -- but in the logic of this Cosmos, it's easier to imagine a tearror gender-flipping Sherman after Tzila was born. If Hieronymous isn't bi, then it would explain why they "went their seperate ways."
(swallows bread, takes a draught of coffee, and grunts, giving you a conspiratorial eyebrow waggle, as if to say, "So wadda ya think?"