If its bothering you as early as chapter 8 when the story has barely even begun you probably wont like the rest of the manga. The unhealthy dynamic, uncertainty, & jealously is the main source of conflict in the story and its what kickstarted the current arc (chapter 83). As for Ryu..he's just a weird guy. He lessens up on the weirdness when Koharu opens up to him more. He's still weird tho. :p
If you don't understand, then don't read it.
I personally love the idea of a polyamorous relationship. It's like being bisexual. You have the capability of loving multiple people (in bisexuality's case, boys or girls) but that doesn't mean you'll shag anyone and everyone you meet.
He is faithful to them. And they are in turn faithful to him. I mean, if you can't understand it, then stop reading and go away. Stop raining on my parade #-.-)
People cringe at yaoi, furries, BDSM, cuckolding, and any other fetish that doesn't appeal to them. Food, books, politics, whatever you're into, I can guarantee there's at least one person out there who would find it cringeworthy. But you want to understand, you want to jump down that rabbit hole. Okay.
First, you are operating under a faulty assumption. A relationship isn't fair just because it's 1:1, tit for tat. What makes it fair is consent.
Put your logic to the test. Is it fair to be tied up and whipped just because the other person will let you do it to them, too? What's missing?
Consent.
Now, do you think a hard masochist wants to be the sadist, or a sadist wants to be the masochist? Even though they both consented to those different roles and are happy, is it still somehow unfair if it isn't exactly equivalent?
If you have a sporting match and consent to a handicap, is it okay to dismiss your loss because it was 'unfair'?
I'm not talking about the "gotcha!" kind of misrepresentations to circumvent informed consent, like a devil's contract. I mean full, honest understanding of what you are agreeing to, without coercion or deceit. Koharu knew exactly what she was marrying into. If she ever finds she hates it and can't handle it anymore, she is free to leave and file divorce. On the other hand, if she accepts it and then tries to force a change, like demanding Ryu divorce his other wives, would that be fair?
Equality is idealistic, but lacks reality. Because we are all different. Just ask yourself: What is equal about only the woman getting pregnant? 1:1 doesn't exist outside of pure math, even on such a basic relationship level. We can do rough rounding to try balancing the imaginary numbers, but if you're busy keeping score in a relationship then you aren't participating.
So, I'll be as honest and respectful as possible. What you do with your life is on you. If you want to marry a man with multiple wives, then go for it. I don't understand it. I cringe at the thought of it. But you aren't me. The only reason I'm even writing this is because I want to try to understand, because I love the art and the character development seems really good, but the most off-putting thing is the whole part where he wants three or more wives to be faithful to him even though it's clear that he is not going to ever in a million years be faithful to any of them. In chapter eight, he says "just thinking about koharu becoming my personal property sends me aquiver" and I stopped. Because you don't say that to people unless you return the favor of becoming their property, and even then it still seems like an unhealthy relationship. I don't want to judge. I'm not a jealous person, I just feel like monogamy is either all in or all out. You either date one person exclusively or date many people with no intention of being faithful or expecting faithfulness. Please explain. Will I ever get it? Can I ever continue reading? do his wives ever make out??? No seriously, can they do that??