Chill out, bro, it's just a game hahahahah
Yes, it's a game. A game with a story that could happen IRL. If happened, I'd say the decisions of this guy are stupid. He feels stimulated, but he also suffers. And still he just decide to put more wood in the fire! And, after all is said and agreed, he goes and "oh, what will happen when Kenji leaves, will we be able to come back the way we were", without noticing that since that trip to the beach they barely live as a couple anymore! It's like the couple is Rina and Kenji, since she spends almost all her time and attention on him, even without sex involved. And for him as well, he won't be able to just go back. Fetish is like a drug if you don't control it, and he didn't!
Of course, like I said before, anything can happen before the end. I hope that, despite all the events, that there's a way for them to keep being happy together, with dignity and respect. After all, I like happy endings! You can ask: "then what are you doing in a NTR game forum"? Well, since it's a game, and not real life, we can expect things to work out well, despite going through a rough path until get there, since in a story anything can happen!
But, in the case of this couple, I think that they are both already too far gone. Only through a very difficult process, maybe even traumatic, where their love can keep their bond strong, and the right decisions are made, they can be just a couple again. Or, they accept the changes, and go on with their lives as a "cuckold couple". The best would be "swinging couple", that's an excelent path as well! But boundaries need to be restablished for this to work, so that both can enjoy this new life, without fear or insecurity, while still loving each other and remain making sense as a couple.
And there I go, running my writting again! What I mean is: I know this is just a game, but we all have differences between our immersions. Apparently you, when finish reading a book, just close the book and take another to read. In the other hand, I like to pick up details of the story, theorizing and trying to find alternatives for the plot developed. Specially in NTR/NTS stories, that get me really into it!