In many countries it is illegal to ask direct questions about the candidates' age, marital status, childcare requirements or political affiliations. If you are asked an illegal question, pointing this out will run the risk of alienating the interviewer. It is best to determine what the interviewer's real concern is and then to reassure them that that area will not interfere in your ability to do the job.
In countries where such legislation exists, certain subjects are considered protected. However, the subjects can be broached if the question is worded correctly. You almost need expert knowledge of what is the correct wording before you can claim that a question is illegal. Protected subjects include race, religion, gender, marital status, family status, sexual orientation, disability and national or ethnic origin.
If the interviewer persists in asking illegal or improper questions, this may be a signal to you that this is not the right employer for you. You can point out that you believe the question is illegal, but you had better be sure of this. You will alienate the interviewer nevertheless. Alternatively you can comply with their concern especially if you do not believe it be detrimental to you. If it is detrimental, than perhaps it is for the best that this issue is resolved now rather than later. Proceed with the rest of the interview as professionally and politely as if these questions have not been asked, but keep them in mind when assessing this organisation.