These days, marriage often serves more as a merger than "'til death do us part" marriages of, say, 30 years ago. The economy plays a part in it. With women earning a wage to support the family, there is not the sense of traditional go-to-work husband and stay-at-home wife. Earning their own money, especially in two-income families, means they are more likely to claim possessions as solely theirs.
More women are going into marriage knowing more than half of marriages end in divorce. They keep a certain detachment and hoard possessions for when the end does come.
With the ease of divorce, most couples will rush into marriage to suit parents and friends, knowing when the novelty wears off, they can end it. The endearment of "husband and wife" never attaches.
In the politically correct state of the Union, some couples choose to call each other partner in support of GLBT rights to marriage.
Whichever answer applies to a specific couple, they are all signs of a paradigm shift of the society's view of marriage over time.