In short, the enlightenment privatized marriage, taking it out of the public sphere, and redefined its purpose as individual gratification, not any 'broader good' such as reflecting God's nature, producing character, or raising children. Slowly but surely, this newer understanding of the meaning of marriage has displaced the older ones in Western culture.
Proper teaching is recognized with ease. You can know it without fail because it awakens within you that sensation which tells you this is something you have always known.