He's checked out because he's had enough. I didn't see the very beginning, so I don't know if he admitted to having an affair or being with another woman. I am NOT an advocate for affairs. But, research states that spouses (of both sexes) leave or have an affair because the way the person they are having an affair with makes them feel. If he got some respect, some tenderness, a person who actually listened to him, made him feel special and not simply a servant for the kids, was kind instead of critical, yeah, I can see why he finally checked out of his cold, emotionally empty marriage.
It's a shame. I saw a couple pictures and footage of Kate at the start of their marriage. She was actually softer. She's become cold, harsh & critical. There are parents who've raised large families without becoming that, so having large families is not an excuse.