Men are from California, women are from New York?

Via Feministing, I discover this map from National Geographic indicating where single men outnumber single women (blue dots) and vice versa (red dots). (Click on the map for a larger version.) Some explanations that have been offered so far:

1. Sexuality -- Gay couples are counted here as unmarried, and areas that are welcoming to gay culture ought to attract similar numbers of gay men and women. But San Francisco, for example, features a big blue dot, indicating more men than women. Insofar as this has to do with the high concentration of gay men there, where are the gay women who would counterbalance this? Maybe lesbians are less reviled by their original communities, so are less likely to relocate to friendlier towns?
2. Age -- Cities to which older couples retire should have a (relatively) high concentration of unmarried women, because there are more widows that widowers, because women tend to live longer. This is probably why Miami is a big red dot. But what about Phoenix, the new Miami for active retirement? Perhaps the mortality gender gap is less pronounced because of the emphasis on 'active' retirement there, or perhaps it hasn't been trendy as a retirement spot long enough to see the effects of age in gender distribution.
3. Career choices -- If men are more willing to relocate for employment than women, and more likely to work in the tech industry, we should expect to see more men in technology growth areas hungry for employees. Denver and the entire West Coast fall into this category. This alone would also account for many of the red dots to the east, but some have hypothesized that many college-educated women are moving east for their careers, and are also likely to postpone marriage.
4. Immigration -- Areas receiving lots of immigrants should have blue dots, because single men immigrate much more often than single women. This seems to be more or less accurate.
5. Lifestyle choices -- Are men more likely to move west in search of fun, sun, tech jobs and California girls? Conversely, are women somehow motivated to move away from same?

Off the top of my head, I'm inclined to think that #3 is probably the biggest driver of the general disparity here, with particular cities bucking the trend for various reasons (Miami due to mean population age, for example).