There has certainly been a major change in lifestyles during the very young twenty first century. We all know the usuals, the visibles: working couples, house chores taken over by technology and so more time to specialize in other things. In other words, since you have more time in your hands as a couple (an educated couple that is) you become much more productive.
According to Mr. Wolfers’ opinion in the NY times http://www.nytimes.com/2010/10/13/opinion/13wolfers.html, marriage has become a luxury, and only a choice to be taken because you share tastes and hobbies with that other someone, so naturally you would want to be together per secula seculorum. He also comments that people without a college degree, although still living together as a couple, choose not to marry because it implies a long (economic) commitment. One that they have not the luxury of getting themselves into.
It makes sense if you put it under the tastes and income perspective, but still, it is a lifetime choice that has many more factors than those. Nevertheless, it is most definitely a trend that we should keep an eye on. Because, if truly marriage is the new form of hedonism, how will we adapt?