They reproduce by budding.
For serious...
Hrothgar reproduce like every other race; the males just heavily outnumber the females, who are kept off the battlefield for the preservation of the species. How such a species survives very long like that I'm... not very sure of, but that's what it is.
In regards to Bozja, like all city-states it was actually diverse. Thing is it had a strict caste system, one of which was a shaman caste which determined who would be queen. Most of them actually were Hrothgar women, but it just so happens the Queen Gunnhildr of legend (who saved Bozja by incarnate summoning herself as a primal with Save the Queen, only to unexpectedly have the Echo which left her lucid and with deific power, so the Gunnhildr's Blades of the time betrayed and killed her so she wouldn't learn she was meant to be nothing more than a sacrifice and go on a rampage with that power) was a Roegadyn.
Doylist answer: we haven't seen Hrothgar women because there are no models for them, and there are no models for them because Viera (p. much).