The reverse of a fictional version of Japan. In a RL version, just reversing chastity roles wouldn't be something so blatant you'd see it at a quick glance, aside from the myriad nanpa practitioners suddenly all being women, not men. A more profound reversal of gender roles would be a bit more obvious, especially in a place like Japan where the historical patriarchy still lingers quite strongly. Yet it's also a modern, highly developed society, so it's not like women wouldn't have all the same rights, responsibilities, and opportunities.
Personally, I find the series with a twisted gender ratio to be more interesting because they have less in the way of "you just have to accept this setting" feeling. They will just make you accept the skewed ratio, but otherwise men and women would act more or less naturally, depending on how out of balance the ratio is. But men are still men, women women, so you don't need to bend your mind like with a series like this. Not to mention, the authors of this type of series rarely bother to keep it consistent.