Clever Elsie has a really thoughtful Singletude post about how to respond when someone asks 'why are you single', pointing out that giving the benefit of the doubt is likely to be a better approach than getting defensive:
As offensive as it can be, though, if you're so inclined, it can be a great opportunity to educate the nosy party about singles. Something I've noticed over the past year and am still coming to terms with is that many people who ask questions like this aren't aware that they're offensive or--hello?--awkward. Sometimes they imagine you must be upset about your singleness (usually because they would be if they were single) and want to encourage you or help in some way. Other times, they may be genuinely curious. They may even just be making small talk and don't know what else to say.
Does that mean it's okay for them to ask why you're single? If it's not okay with you, of course not. But if they're not asking with intent to hurt or irritate, then they're asking out of ignorance, and the best way to combat ignorance is with knowledge--in this case, your firsthand knowledge of being single... As much as you might relish firing back with a real zinger, a candid explanation...could help a singlist person (i.e., someone who has a bias against singles or being single) open his or her mind and understand how "single" can be a good choice, not an unfortunate mistake. It might also help them realize how a question like "Why are you single?" can deeply affect someone. On the other hand, a sarcastic comeback could put them on the defensive or make you seem defensive, reinforcing their unfavorable attitudes about singles.
Her point about people asking out of ignorance made me think about the questions I sometimes get as an Asian-American (for example, 'what are you' or 'where are you from' when the real question they mean to ask is 'what is your racial/ethnic background' ). The person asking usually has no idea how offensive their question is, or that the question itself is rooted in racist assumptions, and every time it happens, I have to consciously remind myself that they probably don't mean to be rude and I should think of it as an opportunity to educate them. At the same time, I have to admit that a big part of me is annoyed that I even have to go through that thought process. That is, why is it my job to educate them, to choose my words carefully so I don't directly suggest that they are racist? Of course I know that the alternative (i.e., saying what is really going through my mind) would be totally unproductive, but it would also be so much easier (well, for me at least). Along similar lines, when I encounter singlist attitudes, I generally try to find a diplomatic way to reply but sometimes I think having to make that effort is, in itself, annoying. Fortunately, it doesn't happen to me all that often. It would be nice to think that we will eventually reach a point where this educating process won't be necessary...