They're always working Even on weekends and holidays, programmers are usually working on some project or another. So no time for dating.
They're introvertsA lot of programmers are introverts who prefer working with computers to interacting with people.
They're socially awkwardMany programmers are awkward in social situations, which can make dating difficult. reason why I'm single :]
They're nerdsLet's face it, nerds aren't always the most popular people in school. And programmers are often self-proclaimed nerds.
They're not into fashionMost programmers couldn't care less about fashion. They're more interested in function over form.
They're not into sportsAgain, programmers are often more interested in function over form. They're not likely to be found at a sporting event.
They're just not that into youSorry, but it's true. Programmers are often more interested in their computers than they are in people.
However, there are also many programmers who are in happy and healthy relationships. The key is to find someone who understands and respects your work.