Over the past decade or so, divorce has gradually become more uncommon in the United States. Since 2003, however, the decline in divorce rates has been largely confined to states which have not passed a state constitutional ban on gay marriage. These states saw their divorce rates decrease by an average of 8 percent between 2003 and 2008. States which had passed a same-sex marriage ban as of January 1, 2008, however, saw their divorce rates rise by about 1 percent over the same period.
The table below details the divorce rates for the 43 states that reported their divorce statistics to the CDC in both 2003 and 2008. It is calculated by taking the total number of divorces in the state that year, and dividing it by the number of married persons, as reported by the Census Bureau. The result is then multiplied by two, since each divorce involves two people. This is different than how the divorce rate is sometimes calculated, which may be as a share of the overall population rather than the number of married persons; I prefer my approach because it will not penalize a state for having a lot of marriages (and therefore more opportunities for divorce). However, there are also more complicated versions of the divorce rate calculation that account for the age of the married couples, and so forth; these are probably superior, but mine is intended to be a simple approach. The table also lists the percentage change in the divorce rate between 2003 and 2008, and the current status of gay marriage and domestic partnerships within each state.
http://www.fivethirtyeight.com/2010/01/divorce-rates-appear-higher-in-states.html