Because I bet they don't ask that question to a man! I was also asked at an interview yesterday "What does your husband think about you applying for this job?". As if it's the 1950s and I need his permission. Again, I bet that wouldn't be a question asked to a man.
I've had 3 interviews recently, including the one yesterday, and at every one I was quizzed about my childcare arrangements.
It really makes me angry.