What I like in this one is that they're acting like adults. Both women never get angsty, or even pondering about "but we're both women!".
Both Kashiwai and Tsuzuki just accept that being in love with someone of the same sex happens and it's perfectly fine. It's refreshing to see characters being adults and giving not a single fuck about what "society" could think. Tsuzuki was already there, being in an open relationship, but Kashiwai quickly caught up. She came out at her job, dumped her boyfriend and was really upfront about wanting to get physical with Tsuzuki.
Parents? Family? Colleagues? Friends? "They'll take me as I am or they can go to hell. For once, I'm in love and that's what's important."
She only felt guilty at first because she was betraying Jun. But on the other hand, we can see that Jun just noped the fuck out at the first hint she might have cheated on him. He didn't ask, didn't try to talk, just ran away. Meaning that he wasn't really in love with Kashiwai either. He was just doing what everyone was expecting of him: marrying, having one or two kids, working hard at his job. Working hard at his relationship? Nope. He'll find another one in no time: men with a stable job are in high demand in Japan.
As for people complaining that it's not a good representation of polyamory, they may be right, because in the end, it's not strictly "polyamory", but an open relationship. One could even say it's a bisexual harem. While Tsuzuki loves all of her partners, the others don't have to love or sex each other for it to work. They just have to accept to share. And it takes adults to do this. Not manchilds like Jun.
And contrary to what people expect, I think we'll get a happy ending and the four ways relationship will work, because Kashiwai and Tsuzuki want it to work. Tsuzuki is free. And that's what Kashiwai loves about her. The end of this chapter shows that Tsuzuki really cares about Kashiwai: making her breakfast, walking her to the station, talking about taking her to the beach, or pool... In the end, she's everything Kashiwai wanted for a partner.
Someone she wants to stay together forever and walk down the aisle with.
It's not fair to him to call it running away. To him anything she does/say would not fix what has happened. You can't put a blanket sentiment of if you really love someone you would try to figure out a problem. Some problems are just deal breakers for some people. Once it occurs and the trust is lost there is no going back for some people.
Just like how you afford the open relationship to the other characters offer the same to a character who considers trust as the main currency in a relationship and once it's spent, it's gone for them.