A group of behaviorally modern humans was cut off from the rest of the earth-like planet. At the time of the separation, humankind had reached a technology level including cattle ranches, brass, writing, and cities. (Same cataclysm, different ark.) About four millennia later,* they were contacted again, but all of them were (or at least looked and acted) female.
I can think of three ways this might have happened:
It takes two women (and no man) to reproduce.
Some vertebrates are sequential hermaphrodites, beginning as one sex and becoming the other later in life. These include clownfish (cue the "Finding Nemo is doing it wrong" nitpickers) and frogs, which inspired Ian Malcolm's famous line in Jurassic Park: "Life, uh, finds a way." Among mammals, some species of mole vole (Ellobius) and spiny rats (Tokudaia) have two X chromosomes, with sex determined in some unknown manner.
Males that look like females
Hyenas are male and female, yet females produce huge amounts of T at puberty, and they're dominant over the submissive males. Feminization would invert this, with males becoming feminized with things like gynecomastia. Male lactation isn't unheard of in the real world; it's seen in Dayak fruit bats, occasionally in goats, and even in humans suffering from galactorrhea.
Males hidden away
Instead of diminishing to near nothing, the sexual dimorphism has become so extreme that only females appear in public. It need not be as severe as the difference between male and female anglerfish, but a pile of sex-linked genetic disorders becoming fixed might cause the appearance of a man to become shameful.
So which of these options is the most biologically plausible and why? Or what other mechanism could make all individuals look female to an outsider contacting this race for the first time?
* This would have to be a rapid change to happen within a 4 ka timeframe, but this timeframe is negotiable. A separation after emergence of humanity but long before the Bronze Age is also acceptable, as the separation date of 4 ka before present may be mythical.