The marriage rate in the District of Columbia is half that of the average of states - a bit over a quarter, vs. a bit over half, of all people. Newsweek blogger Katie Connolly got the explanation mostly right, but her analysis was overshadowed by her comment that D.C. is segregated by apartheid.
Connolly cited three factors that reduce the marriage rate in Washington, D.C. Most of the city residents are African American, and a high proportion of them are poor, the two groups with the lowest marriage rates in America. Second, whites who live in the city are disproportionately highly educated young, Democratic, and women - three groups who do marry, but marry late. Third, the homosexual population of D.C., at about 8%, is double the proportion you would find in a state.
The main reason the D.C. marriage rate is low, which Connolly cites but does not lean on as the main reason, is that it is a city, and a poor one at that. When black D.C. residents marry, have children, and start to move up in class, they leave the city for the suburbs, especially Prince Georges County, MD, which probably has the largest black middle class of any county in America. When white yuppies from all over come to Washington to work on public policy they find a fine city to live and work in - until you have children. Then they move to the suburbs.
Or, if you are really fortunate, and Mrs. G. and I were, you move to Danville, KY to raise your kids.