Not sure about legality of asking but woman statistically (80%+) are main caregivers to kids so asking this question only for females make sense.

It works both ways though - one well known company gives 4 month leave to primary caregiver and only 2 weeks to secondary on child birth. If you are female - you get it by default. If you are father and not single one at that - good luck. Every single person (from your boss down) will ask you why you are primary caregiver.