No one can seriously dispute that the institution of marriage as it operates today, in 2013 in the United States, is materially different than it was in 1813 or 1913 or 1963. It is also fair to say that there have been long periods in history in which the institution of marriage as it operated in practice, was stable for many centuries, sometimes even despite periods of significant cultural change (although, the popular notion of a "traditional marriage" is far younger than most people who employ the term realize, and marriage as an institution has experienced other historic shifts in the past).
Are we there yet? Have we now reached a new equilibrium for the institution of marriage (or are we on the verge of this with the global normalization of gay marriage) that has largely assimilated concepts like no fault divorce, birth control and declining lifetime fertility, increased female participation in the paid workforce, increased education for women, reduced needs for housework as a result of changing technologies and economic conditions, shifting religous beliefs, etc.?
Or, do we remain in the throes of a period of rapid change for the institution of marriage? If so, what changes are left to come? What problems with the institution as it exists, or other evolving pressures, will drive continuing change in it? What would have to change in our society and/or the institution to produce a stable modern marriage institution? Some authors have suggested that female roles have changed dramatically while male roles have changed little since 1963 (for example). Does this mean that the most of the future changes will involve revised male roles that respond and react to new female roles?
How many genuinely different models of marriage are in existence in the developed world today (e.g. Japanese, Swedish, American middle class sometimes serial monogamous, African-American, Mormon monogamous, Muslim polygamous, "traditional" monogamous, American-European same sex), and are more than one of them viable options for an eventual equilibrium state of the institution of marriage?
What are the "moving parts" that distinguish different models of marriage and the ways in which they function in the larger cultures in which they are embedded? Is it possible to set out a fairly comprehensive "model space" of potential marriage institutions that isn't just limited to a couple of dimensions?