Which is not to say that you can't end up with a poor fit anyway. You're just compelled by social expectations (if not legal requirements) to stay together despite the poor fit, often to the detriment of everyone involved.

Sometimes people do give up too easily, and could make the marriage work if they just had a bit more incentive to try. That's where systems which grant marriage more permanence shine. But on other occasions the problems run deeper, and separation may well be the correct answer.

In any case, since people have learned that they can get most of what they want out of a relationship without actually getting married, stronger / less reversible marriages just tend to discourage people from getting married in the first place.