That's what the rational person thinks, but their relationship is very odd. My H is her defacto partner. He's on her checks for instance, though he's been on his own for over 10 years, married for over seven. Out of the three kids, he was the pleaser, the protector, he did everything he could to not make her life more difficult than it already was. This was his role and it has continued, though he's seeing things more clearly as time goes on. My MIL's life was rough because of the choices she made and continued to make, not because it was just all tossed at her unfairly.
Also, since he doesn't have a Dad, all his parental feelings and each parental role is rolled up in this one person who doesn't have a partner. It's a weird relationship.
Me: 42/H: 37 T: 10 years/M: 8 D9, S8 Bomb: 7.23.07 Separated: 1.20.08 D Final 3.19.09 Affair started in '05, found out parts in 11/07. They married 11.26.09