Interestingly, this only appears to be true in Manhattan. In New York City, single men aged 20-34 outnumber single women. And according to this site, single men aged 20-34 outnumber single women 20-34 in almost every city in the US: http://jonathansoma.com/singles/

I have several hypotheses for Manhattan, but my best guess is that a major contributing factor is that the average age of marriage (and proportion of never-marrieds) is higher in Manhattan than outside. http://nypost.com/2010/01/05/all-the-single-people-in-manhat...