So that's the obvious factor. Let's call that Equation A. But there's another factor too, I think - we'll call it Equation B - whereby those teeming hordes of urban singles KNOW that they are single, and that they live in the city, and that they have a seemingly endless array of potential partners to choose from. So, is the urban population choosier? More fickle? More jaded? To the extent that the urban single population is self-selected, is it because they're looking to find a serious relationship, or looking to avoid commitment?
And beyond Equations A and B, there's just the sensory overload of living in the city. Even if you live in New York and love every minute of it, you must know what I'm talking about. How does that impact our ability to form and maintain relationships?
And finally there are the reasons people live in cities, which may overlap with Equation A but are not identical with it. There is, specifically, the "safety in numbers" principle. If you are an ethnic minority, a political minority, an artist, intellectual, or queer (there's the overlap with A), you want to go where there are others of "your kind". It's sort of the general case of Equation A.
How do you balance all of these things - especially when your politics and your values are a weird mix of small-town (read "red state") and urban (read "coastal liberal")?
I don't know the answer. When I get it figured out, I'll let you know.