Marriages can have patches like this. All it seemed to have been is that she has lost sexual intimacy with him which caused a down turn in their relationship. Nothing that can not be overcome.
in chapter 3, the wife is talking to her cousin (MC's coworker) on the phone.
Her cousin mentions how MC is currently a project manager and is being relied on more and more at work of late, and the wife, unprompted, just replies with "aren't they just making fun of him?~"
and she says it with a smile on her face while laughing.
This reads as way more than just her "being turned off sexually" by her husband. She literally kicks him when he's on the ground & in the middle of their floor after his first time at the gym, yelling at him for being slow and in her way, not once asking if he's actually alright.
She constantly admonished him for his appearance and age and "lack of usefulness" throughout their interactions in the first two chapters.
They have separate beds. Sure, that could be due to differences in sleeping habits, but on top of everything else and the fact that she shuts him down with a "you're an old man, stop acting like this" when he tries to be intimate with her, clearly shows she has no qualms about shutting him down and shutting him out, or having actual concern.
And the MC tries his best, at home and at the office, and
everyone walks all over him in both places, including his kouhais, which is extra telling given how important hierarchy is in the Japanese corporate social scene.
If it's a matter of "their marriage is having a rough patch", it's pretty clear from the first three chapters at least that the wife is the problem here, with the only apparent issue on the MC's side being his lack of confidence due to all the putdowns and neglect he's received simply because he's a middle-aged man who isn't super physically fit.