This is actually a nice read because it's not your usual fluff and sex. There's a good plot around the cast regardless of the fact that people are cheating on each other, but in a way that's also very real and raw.
Edit:
When I read through this the first time, I took the development at face value and missed out. Yes, cheating is wrong, communication is a must especially during marriage(s) at this stage.
However, the author has written quite a good story portraying how you can fall in love and fall out of love as well. After rereading the series' available chapters again, there's multiple dynamics that you could/should consider to obtain a better understanding of the characters. I love the writing so far and am honestly excited to see how this ends.
Michi is portrayed as the "simple" wife. She can support her husband, is diligent about her work, and more. She's normal as you can get which isn't a bad thing at all. She just simply wants to try to enjoy what she has which includes sex while married with someone she likes (or liked in this case due to how it's developing). She finds that solace in Niina who's able to mesh with her very well (and has shown us readers that he's a very capable husband). Unfortunately, it just teeters upon cheating on her own husband.
Youichi/Husband is shown to have developed a deep/severe case of apathy as a self-defense mechanism. When we got his flashback, he's shown to be incredibly simple-minded and is willing to be straight with you/no beating around the bush. For someone who's so used to viewing the world as black-and-white, he can't quite adjust well when someone is trying to throw color in. Ultimately, the story is pointing towards him breaking off the marriage because he was either never interested in the first place, only married out of convenience, or doesn't want to deal with it. While he's acted as a faithless husband, he's pretty pitiful. Of course, this doesn't justify his actions and cheating at all.
Considering the two's dynamic, it looks to be a very dead marriage. The two don't suit each other well.
Now we go on to the next few characters that are introduced.
Mishima is someone who's suffered from a lack of communication and abuse. She's tired of being taken advantage of and just wants things simple if possible (unless I missed something somewhere). This is why Youichi was pretty much a "fresh breath of air" to her. He never showed much of an ulterior motive, and was straight-forward in communicating. She's treated more or less like a complete side-character to add a factor to the main couple's conflict, but maybe we'll see more. She's a solid character for where we currently are in the story. This is where the author is showing that Youichi and Mishima have a pretty good dynamic together. It doesn't necessarily mean they'll be lovers, but can definitely be friends.
Niina is the capable husband, can be loving, but falls out of love if he doesn't receive any back. That's pretty standard. When you're married to someone you love yet they don't reciprocate in some way shape or form, it can get hard since the foundation behind that piece of paper (marriage certificate) is the love between two individuals. You can only give so much love until you run out, especially when you don't get any back. On top of this, he's paired with Kaede who's an extremely independent woman that clearly knows what she wants and works for it. While this isn't a bad thing, it's obvious that she more or less neglected Niina and arguably took advantage of him/took his kindness too lightly. He doesn't mesh with with Kaede, which is why I suppose he went to Michi. They both find solace in each other which is nice but is still wrong since they're both cheating.
Finally we have Kaede, as aforementioned, incredibly ambitious, independent and works hard. However, this mindset has clearly made her tunnel-vision out of control. She realizes late into the game that she neglected her life and that she forgot to separate her work life and personal life. This is further implied in the recent chapter when one of the employees left early and she commented on it. In all truth, the author gave her the short end of the stick. She was not necessarily ready for marriage since she's so work-centric, and she had other goals she wanted to pursue. As much as they may have loved each other back in the past, her marriage with Niina was too soon. Again, she realizes too late and is conflicted because she wants to patch things up, but unfortunately has been too unaware to how many holes there are that the ship has nearly sunk already.
There is no dynamic between the second married couple. As sad as it is, you can tell when Kaede's trying that she realizes her own shortcomings and Niina is yet again reminded of what he has dealt with so far. It's also an essentially dead marriage.
I just find this cast to be beautiful because there's different perspectives coming into play. I'm surprised this series only has the Seinan tag. It should also have a Josei tag since the content of this story is very well-rounded for all audiences.
Anata ga shitekurenaku temo