It is unfortunate this was asked because it biases decisions that both the applicant and interviewer might make. Medical school interviewers are actually forbidden to ask these questions, and this interviewer would get into trouble if you made some noise about it.
I went through medical school married, and had three children by the time I was out of training. Hand on heart I will tell you that this career absolutely needs a supportive partner, or the relationship won't last. It takes a VERY secure/selfless partner to weather the training period and/or (hopefully and) a very determined student to shield them from it. I've seen many colleagues with high maintenance partners absolutely struggle when they are made to feel guilty for working. This will be your career, vocation, and hobby for the rest of your life, I promise you. You'll be studying constantly for the next 10 years (the MCAT and medical school are merely the tip of the iceberg; I'm 11 years in and I still have exams this week), you'll be gone a lot, and you'll be tired when you're home.
The best advice I can give you is to make a special effort to recognize your partner's effort at every point--let them know they are appreciated. An applicant is accepted to medical school, but a partner (and family) experience everything the trainee will experience.
Edit: the location of my medical school, residency, fellowship, and first professional posts were all chosen with my partner's preference. That was the least I could do.
Best of luck!
My spouse is in business and his schedule is already incredibly difficult and time-consuming. Some exception there with weekends, but even then, he's inundated with emails of "urgency" regularly. I believe busy careers mean that couples/families have to go the extra mile to make things work--a lot of give and take.
Long distance relationships are mostly a big issue for couples, and reasonably so.
If kids are small, well, you better be able to afford a live-in nanny or something like that.
I will not be able to move 3,000 miles away to attend MS. It just wouldn't make sense for at least a dozen reasons. As a non-trad in a long-term, committed relationship, we have to be reasonable, but we also have to be willing to make some compromises. I don't really look at this question as much different from if you have a child, and say, for IDK, a court-appointed reason, the parent could NOT take the child away to MS with them. So, then what is the parent going to do? The parent is going to apply to schools close to where the child must stay. When you are married, you have started a family, period--children or not. At least that's the way I look at it. I made that commitment well before any other, and so did my husband. There's no guarantees on anything, and I do know of some long-distance couples/families that somehow manage to make it work. I think they are the exception and not the rule. I also know a number that have tried it, and the "experiment" was a miserable failure.
About them asking the question in the first place, this sounds quite familiar to another question that was deemed inappropriate by people and would also be deemed as unallowable for a job interview--at least within my region. I think if the interviewee mentioned the situation in the first place, they can get away with it--at least apparently med school adcom people can. I have no idea why they are allowed to get away with it, but whatever. Sometimes they do ask questions that are on the side of things that I would, out of an abundance of caution, just move past--such as focusing on gender or sexual orientation or religion or marriage/relationship status. Those areas fall under the "It's really no one's business unless I make it so," and "Do you ask the same question of those of the other gender? " Safest bet is to avoid these kinds of questions altogether, but apparently according to some people here at SDN, adcoms get some extra leeway it seems.
The original question or even similar ones are not necessarily bad questions to ask, it's just that to me, they should come before an applicant has interviewed. I mean, it's a good reason why there are places like SDN to discuss these things ahead of time and reflect on them. I just don't think they are appropriate for interviews b/c they cross boundaries--and it's really not the interviewer's job to play, "Have you thought about how this is going to affect your personal life or whatever kind of questions?" By the time the applicant reaches the interview process, this should be settled, and it's not for anyone else to step into those personal boundary areas at that time. The applicant has made their decision--even w/o the fullness of deliberation that they should have taken. If they have not, that is something they will have to work out. You will not have every applicant sort out every thing in their lives in order to evaluate if they are perfect or near perfect or better applicants than others. It's not a therapy session, it's an interview. You don't go into personal areas during professional interviews, and this should not be any different from graduate school application. Employers stand to lose money too when they have applicants that have unresolved personal issues than can affect the job. In fact, they aren't being paid to take on the employee, and it's a lose situation for them often enough when there are such conflicts. Nevertheless, employers are not permitted to get into these areas upon interview.
Likewise, when I apply to residencies next year, my rank list will be heavily weighted towards where my partner would like to be. 
