They've succumbed to the spirit of the world.
It's true that couples who are living together and have a proximate wedding date set and a deposit put down on the reception experience fewer bad outcomes than couples who just live together, and either break up or eventually get married, maybe after a couple of kids.
However, that doesn't change the fact the "married" is "married," and any relationship can be considered "stable" right up to the moment it ends!
Even in this day and age there are people who marry because they’ve come to feel ashamed of “shacking up”.