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One of the 5 key principles of Fair Employment Practices according to TAFEP is:
"Recruit and select employees on the basis of merit (such as skills, experience or ability to perform the job), regardless of age, race, gender, religion, marital status and family responsibilities, or disability."
If the interview seems to be straying into areas where these appear to be deciding factors, you can respond with:
"Why do you ask?"
and get further clarification.
It's not immediately clear why they would ask such a question, so I'd give a potential employer the benefit of the doubt as to whether the line of questioning (and criteria of hiring) is appropriate, and let them explain their reasoning. It could be due to hazardous work conditions, for example, which may be a legitimate concern.
If they don't have a good reason for asking and respond with something like "Just curious.." then it's reasonable to say "That's a personal question which I'd rather not discuss."
In short, be polite about it, but don't feel pressured into answering something that doesn't make sense to you, unless they can clearly explain why it's relevant to the job.