I hope I'm not insensitive, but if I'm having a bad day and my cab driver (or just about any total stranger) asks "how is your day" I am not compelled to tell him about the intricacies of being me. It's not about fighting for "the truth", it's simply practical. This is generally how to behave for all matters of sex, politics, religion, and anything else personal. Small talk is small talk.
I don't mean to be presumptuous, but are you straight? Not feeling compelled to tell people about your sexuality is a luxury afforded to people who fit into the norm. It's what LGBT activists would call "privilege", and it's something that we as straight/white/male/other dominant group need to be aware of if we don't wish to alienate minority groups.
There are plenty of other reasons you might have kids but not a wife. Maybe you're a widower. Maybe they're actually your brother's kids, but he and his wife were killed by a drunk driver, and you're the godfather.
Perfectly legitimate cases where you just have no reason to get into it with the freaking cab driver, and probably don't get into a serious conversation, and probably also don't tell him to mind his own damn business.
Wouldn't it also be inappropriate for the cab-driver to follow up, "Do you have kids?" with the question "Are they biologically yours?"