Wow - Momof4 got a little nasty there.
It doesn't matter what the employer WANTS to know. It matters what the employer is ALLOWED to know. How dare anyone assume that someone with small children is less reliable than someone with a million other issues.
You were right about marital status and year of birth. The employer might want to know if you are 35, a Republican, a Methodist, an Italian, and someone with a cholesterol level below 200, but it's not his business.
So, is your question "What should I have said?" - I might have paused long enough to make the interviewer a little uncomfortable. Usually people fill the silence. My next question (if I'd had the presence of mind) might have been, "I'm sorry. What is it that you are asking exactly?" Get them to elaborate and maybe think about it. Then you could have said, "I don't understand why you need to know that. Is that a qualification for the job?" Then add, "If you are asking if there's any reason I can't be at work regularly and on time, the answer is no."
If you can get a copy of the application, send it to your state's office against discrimination. The doctor's office needs to update its applications to comply with current laws - and they can easily get boiler plate applications from any state employment agency or half the office supply stores in the area, and on line.
The questions they ask you have to be relevant. For example, they can't ask you your age, but they can ask, say, in a bar or restaurant where you would be serving alcohol, "Are you over the age of 21?" BECAUSE IT'S RELEVANT and the LAW. I'm not sure, but that might apply in a doctor's office if there are medications you would be dealing with - but again, you'd have to check on that. They can ask if you have a valid driver's license if the job requires driving.
If you're going to be working there, you'd best report this because if YOU are asked to have anyone fill out the application, you don't want to be particpating in this!