its tough. like the early chapters seemed like both daughter and mother were pretty shitty to him. And things are only changing because of the girl. Its tough. WIthout ayase, he would still be miserable, but I dont think cheating is what we want to see. BUT AYASE IS THE ONE PUTTING WORK IN for the mc, sounds like she should reap the rewards! SO conflicted!
I mean it looks like both the husband and the wife are not that passionate about each other and kind just rolling with it. The cousin's flashback is also from way back since he appears to be a high schooler in it. So those days are long gone. Only reason she is paying attention now is because she is suspecting that he is cheating.
She has probably thought more about the possibility of him cheating in the last couple of chapters than she's thought about him as a person in the past decade. Like someone else mentioned, an agreed upon divorce would not be that weird of an outcome for all this.
Regardless though, MC's problem is more fundamental as in, he has zero agency in whatever he does. Just this little tid bit of how the cousin invited himself in even though it could have easly been a massive disaster, he couldn't stop him. If there will be any improvement, his doormat persona has to go first.