I qualify that as being a question that parents will ask, because rarely is it how a person who doesn't have children will feel the need to strike up a conversation. And sometimes it's the question those of us who are childless/childfree least enjoy hearing. Why? Because answering with a simple "No" can then lead to an awkward, uncomfortable silence as the parent struggles to find anything else to talk about. Or it can lead to the even more uncomfortable, intrusive question of "Why not?" or inanities like "Oh, well, you've got plenty of time yet" - which might be completely untrue, or makes the assumption that the childless person actually wants children in the first place.
It can get even worse if you are childless for reasons not of your choosing—say because of infertility. You may try to shut the conversation down by admitting you have fertility problems and wish you did have kids, but it doesn't seem to be working out for you. While you'd think such an admission would get a stranger to realize this isn't something you want to talk about, often it just seems to end up leading to lots of unsolicited advice and suggestions. Someone who was a complete stranger just moments before now may feel perfectly in their right to make personal, intimate suggestions like, "Well there's always IVF! That worked this woman I know at work" or "Maybe you just need to relax and stop thinking about it."
I love this animated video which satirizes this kind of aggravating conversation perfectly: