Okay, first...
Bro. BROOOOOO.
You are over the age of 35; you "can't stay calm" in a mixed hot tub? Last I checked, you were Japanese, and I assure you: Japanese dudes are
entirely able to stay calm in a mixed bath.
It is difficult, I will admit, between the ages of 12 and 16-ish; I've been in mixed bathing with young, middle aged, and older men and women... nobody had an issue.
I've also been 35; hell, I've been every age before 45! You can stay calm, unless you are mentally
and physically under the age of 18.
Which you are not.
Please, for the love of Amaterasu,
STOP PRETENDING JAPANESE ARE MAMMOPHOBIC INFANTS!!!
Fucking christ.
Okay.
Second:
Why are they forming a monarchy instead of republic
It's the "next step"
We know humans have difficulty with governance, and we have learned that if you start with religious theocracy (which is what they have, albeit a gentle form of it), the next step is to have a monarchy.
As people start to identify with the idea of being a single political grouping, the need for a monarchy wanes... but this is only once they all identify as a single political group! Only then, can onr have the citizenry move toward a representation-based form of government.
This is done usually starting with a parliment/congress; having the people look to a "Baron/Representative" to directly address the people's wants/needs, and then having a "Count/Duke/Senator" who does what they personally believe will help the people.
They them bring those to the "King/President" who makes the final decision.
The big difference is primogeniture vs election (are they born as a "Baron," or elected as a "Senator?"), and as people move forward in society they become aware that they are dependent upon the society around them, and thus more invested in seeing their neighbors succeed (a rising tide raising all boats, if you follow).
But it all starts with a monarchy or dictatorship to cause the people to see that they are all part of the same sociopolitical group.
♤□●■○♧□●■○♧□●■○♤
[A SMALL DIGRESSION]
There are, obviously, regressions; as some people lack education, or they are simply not intelligent enough to understand the concept of others being equal to them, you will find people calling for dictatorship.
This happens because they don't see the world as "real;" they are still mentally an infant or small child, and therefore believe that if they find a magical sky daddy to make all their decisions for them, it will "make it all better," somehow.
Mostly, "make it better" means that they can play all day long and never need to act like an adult; never have to listen to people, never see them, never think their own thoughts, never feel "wrong."
The demands for dictatorship are made by mental children, and infant egos... but they are out there...