DC is apparently packed with single men and women, according to the Pew Research Center. With a national average of 52% (men) and 48% (women) married, the District sports a mere 28% (men) and 23% (women) married. The next lowest numbers are 47% (men) for Alaska and 43% (women) for Rhode Island. (These numbers are for the 15 and older crowd.)
One commentator believes this is due to the unique demographics of DC: very high black population (less likely to marry) and very high Democratic population (more likely to marry later). Another points to the 8.2% gay population as the culprit (along with the marry later point). These are certainly factors, but there's a much more obvious reason that I think carries the bulk of the explanation.
Married people tend to want to start families which generally means a bigger home and unless there's also a big raise, that means moving to the suburbs. In most states, moving to the suburbs can but not always means changing your state. But in DC, it always means leaving DC and heading to Maryland or Virgina (or West Virgina). So the states include both the city proper and the suburbs but DC includes only the city proper. Big difference.