I think what someone said about re-directing the question is good advice if you can do it, but might be difficult to pull off without being obvious. However, telling the interviewer the question is illegal or answering it honestly may count against you in the interviewer's eyes, whether they will acknowledge it or not. I was just reading something on MSN about this, it said to address the underlying concern behind the question without answering the specific question. For example, in the workforce, the concern with a woman getting pregnant is that she will leave; so if a woman is asked a question regarding future parenting plans, she could respond, "Whether or not I have children is irrelevant to my future career, but I plan to work and have a career regardless of what happens in my personal life." It is a little trickier with adcoms, because you have to decide why they are asking the question, then try to address that rather than the question itself. They may have the same concerns as a job inerviewer would, or they may be trying to find out, as someone suggested earlier, if you have given serious consideration to your life plans; or have some other reason altogether.
An interviewer at a med school in state A might ask you, for example, "So, what will your boyfriend/girlfriend/spouse who is attending school/working in state B do if you are accepted here?" The concern that the interviewer is probably trying to address is that you may not attend their school if admitted due to relationship constraints, and they may probe into your relationship (illegally) for this reason. Alternatively, they may be trying to make sure you have given thought to this issue, regardless of your response. You could address both of these concerns without actually answering the question by saying, "My partner and I have discussed my plans to attend medical school; he/she is comfortable with the process and knows how important pursuing this goal is to me, and that I will attend the school best suited for me."
Of course an interviewer could always keep probing after you give such a response, but if someone is showing such blatant disregard for propriety in the interview after this point I don't think you would be out of line to politely but firmly point out that questions regarding yout personal life, religion, sexual orientation, etc. shouldn't be relevant to your admission to the school, which is after all the purpose of the interview.