The thing I think is HUGELY unreasonable is staying together then divorcing when your child goes to uni or is a young adult. I have known 5 or 6 friends whose own relationships and feelings were so damaged by that happening
My FIL left my MIL as soon as he felt his kids were adults and ‘his job was done’. It was enormously selfish, came at a huge cost to everyone, frankly - even now, decades later.
My MIL didn’t see it coming. Then it transpired he’d been unfaithful regularly throughout their marriage, including with a family friend. My DH and his siblings had no idea - to them they’d had the perfect family upbringing, their dad was wonderful, and now it all felt like a lie. Their mother fell apart and because everyone was an adult no feelings were spared, no emotional drama not laid out there.
Like the quoted poster, I know of others too who feel the same.
Maybe you’d feel relief if your parents were unhappily married, fighting, tense atmosphere etc. But if you’re happy enough to ‘stay together for the kids’ and parent harmoniously and look like you have a proper, functioning couples relationship, it’s a massive shock. You’ve effectively lied to your children - you’ve said ‘This is a happy relationship’ when at a fundamental level it wasn’t.
Stay ‘for the kids’ if you want to, but he aware what you really mean is you’re staying because it’s easier for you.