Stories like this would be less creepy and weird if they were just set in college.
Wow, what a good point. It would be really helpful in lot's of ways
That's a cultural issue, though. It is, sadly, a fact that high school-aged compensated dating is a very real thing in Japan. The main event that started the plotline was when the MC who insists that two girls can't be a legit couple was stating that she might give that a try to make more money, probably in reaction to the fact that she had been sexually harassed at the part-time job she had just quit, now that we have the full details about that. Sort of "If I'm gonna get groped on the job anyway, I might as well get paid better for it and have at least a chance of choosing which clients I want to make money from."
So far, at least, there hasn't been anything in this manga that has been nonconsensual, no matter how much the lady doth protest. She agreed to their bet and the terms of the bet included physical intimacy of the type that has occurred. In addition, the type of things they have done are not out of the ordinary for people of their age group in a relationship. Indeed, since they are 2nd year high schoolers, they are old enough that they are almost old enough to legally buy the porn they were watching. Yes, they are not technically old enough to watch porn, but, let's be real here... how many people who WANT to watch porn don't find a way to watch it before they are legally of age to watch it?
The next arc, showing Fuwa working at a lesbian bar as a 17-year-old, might be a bit more sketchy, but she's already stated that she's legally allowed to work there, but only until 10 pm and only if she doesn't handle alcohol. I presume she's working as a hostess, greeting guests and chatting with them if they request her company. This is another legal form of employment in Japan that, I'm sure, many people will find squicky and think is wrong, but it is a valid form of employment and looking down on the legal sex trade, especially the low end of it where there isn't any actual prostitution involved, is something that the Puritan founders of the USA would be ooooh so proud of! Right along with condemning gays and lesbians! There's not really any valid reason to do it. This isn't the TV hookers from 70s and 80s TV shows. It's entirely different.
Ask yourself: Would two years really make THAT big of a difference in this story? Were you REALLY that much more of an "adult" when you started college than you were your second to last year of high school? (My personal answer is NO! I am pretty sure I regressed in maturity when I went to college and didn't really recover to the place I was as a high school junior until about my junior year of college!)