That's true, but even if everyone you know does have kids, that doesn't mean you're invited to baby showers every week. Especially if you find the whole subject of children upsetting, as the author seems to, you also have the choice of mailing a baby gift, or simply declining an invitation, no further discussion.
I simply can't think of another situation where, "Do you have children?" is an obvious question to ask a stranger. Can you?
You get invited to showers for old high school or college friends, folks at church, folks at work... It's not uncommon. As for the "do you have children" question, I get asked if I'm married a lot. It's just a typical smalltalk question. I don't take any offense to it. It's sort of like "where do you work" or "how long have you lived here". It's just a way of finding a conversation topic. I've gotten that at office parties where you meet the spouses, at church stuff where you're meeting new people, and at work when getting to know new people. Where I work, you work with small groups of people for years at a time. So, you tend to get to know each other pretty well. Whose kids play what sports and how their team's doing. Whose wife wrecked their car last week. Whose husband avoids doing the dishes at all costs. :) That sort of stuff.
I ask that question whenever I go to my husband's work functions. I'm looking for other mom's to talk to. I have little in common with the women that work at my husband's company, so I end up trying to find other moms to talk to. Even then, it's hard to find another stay-at-home mom.