The girl dungeon is literally a girl, too, btw. She looks man-made but shes actually an enormous organism with a symbiotic/predatory relationship to adventurers. Her entire ecology is estrogenized, and a lot of those monster girls are basically white blood cells. Every "cis male" adventurer we send down gets turned into one so thats why the guild exclusively employs monster girls for exploring her.
Trans mascs on the other hand, hoo-wee. Dangerous place to be outed in. Her architecture will treat you as a foreign contaminant and try to eject you like a whale shark's stomach, theres no telling where you'll end up. Thats why so many rescue teams are sent down there, lots of "cis women" get sniffed out as boys and cause an immune response.
Much easier to just repurpose those "boys" who keep coming back as monster girls, yeah...