@Sogeking79:
The Japanese are pretty law-abiding. Prostitution
is widespread in Japan, and brothels make a lot of
money, so there's no reason to invite trouble with
the authorities. Besides, it's the girls who pay the
fines (as seen in Pretend Strangers).

@Sogeking79:
The sign is serious. By allowing Sensei to fuck her
(bareback, no less), Mitsuki is knowingly breaking
the house rules.
"I'll do something that we really shouldn't do."
(page 16, panel 3)

@Sogeking79:
In Hatimoto's Selling My Wife, Misako also knowingly breaks the
house rules by allowing her hubby to fuck her (as a special favor).
Note the "Prohibited Actions" sign on page 4, panel 2.

Yeah, but his total at the end was 60,000 not 160,000. So the establishment only has the sign to appear legal while letting their girls go all the way.

Also she said she had never done bareback before. She may "work" off yhe clock, but she didn't seem the type to use dating apps. If she did, I assume she would make more than at a massage parlor and just do that. If it were sex with a boyfriend, occasional bareback would be normal unless there is great fear of pregnancy. So we can safely assume she only has sex at the parlor and they allow it for extra revenue. In the worst case they may allow it and still fine people fir that extra scoop of cash.

I read the "something we really shouldn't do" as her trying to tempt him. She had been extolling the virtues of villiany and (bareback) sex with a student was one of them. Unless she meant "Let's risk my job security, that's super hot! Nothing gets me wetter than the thought of job hunting. If I have to explain why I left this job, I'll flood my panties!"

On the other hand, what teenager thinks that far ahead.

I don't know fir certain how serious the signs in Japan are or how strictly they are enforced. As an American I see the sign as something like "Employees must wash hands". I assume most people are following it, but when someone doesn't no one cares unless an inspector or regional director is there.