I am a female applicant and was asked whether I was married all along the interview trail. Granted, I was wearing my ring, so it was more of an observation and not a question. I completely agree with BoneBlaster on this one. I think this question is an attempt to get to know someone on a more personal level. Usually, the follow-up questions is, "what does your spouse do?" "Where is he from?"
Let's face it, talking about research and your AOA status is not going to reveal your personality. Five years, 80+ hrs/week is a long time. I hope I match at a place where they tried to acquaint themselves with each applicant personally, hopefully, they did their due diligence in finding a group of applicants that I will be happy to be with for the next 5 years.
Also, "Are you married?" "Nope." I think that would end the conversation.