If they have kids then you're tossing in a whole new set of variables. Can you break up with the adult without losing the relationship with the kids? How realistic are the options of maintaining the relationship with the kids? Etc.
Depends on the person. Some wouldn't care about the relationship with the kids, but they might care about the child support payments.
Regardless, it's understandable that more people would endure a bad relationship when there are kids involved.