Don’t. You seem to be utterly blind to the enormous power imbalance in an interview setting. If you were to ask people what their sexual preferences were, or what their political opinions were, people would feel compelled to answer. Don’t behave as if you don’t have any power and are just talking informally; you’re not.
You have a huge amount of power in that relationship, and if you will be their boss after they are hired, this will continue to be true. Wield this power responsibly, don’t pretend you don’t have it.