Are you planning to start a family? / Any variation of this.
I have a bout of job interviews coming up so I've been doing some practice with my dad who regularly recruits staff.
He threw this question in there when we were rehearsing. At first I actually laughed, and then told him that he wasn't allowed to ask that. That I was pretty sure it was illegal to ask that in an interview. Anyway to cut a long story short he advised me that the only acceptable answer in this scenario would be to say that I have no plans and move swiftly on.
He said that if I refused to answer the question, said yes or called them out on/argued about the questions legality then that would not help me get the job. Which really is the overall aim.