As part of your introverted correspondent's mission to understand social conventions, I find myself interested in what personal questions are considered polite and impolite to ask.
One that I find particularly odd is the fact that sizable numbers of people seem to think it's appropriate to ask newly-wedded couples when they're going to have children.
I think this isn't considered impolite by a lot of people, but perhaps ought to be.
Now, this isn't in the category of easily disclosed facts that needs to be concealed in order to prevent social friction between the questioner and the respondent. An example of this kind would be how much someone earns - there's not much ambiguity about what the number actually is or what it means, but disclosing it might provoke either envy or embarrassment. I'm not a big fan of those types of taboos, but I can understand why they exist.
The problem, rather, is that the question seems quite likely to be unresolved, and possibly a sore point as well.
Let's assume your early thirties married couple hasn't had children yet. There's a range of possible reasons that this might be the case, and a lot of them suggest you probably shouldn't have asked.
1. One of the two parties wants children and the other one wants them either later, or not at all. This is almost certainly likely to be a mildly sore point (at a minimum) between the two, and likely not an argument that they would relish re-litigating in front of a public audience.
2. They're actively trying to have children, but are having difficulties conceiving. Way to go! Their medical issues are undoubtedly something that they'd love to talk about at the dinner party. As a bonus, you can also delve into who might be at fault between the two of them! Is he shooting blanks? Is she barren? At worst it's hugely awkward, at best it can publicly reopen rounds of hurtful recriminations!
3. Neither of them want to have children yet, but they'd rather not explain this to you. This is the most compelling reason to not ask, that for the vast majority of people on the planet, it's simply none of their business. Having children or not seems like a fairly important and personal consideration that lots of people might not want to discuss in front of everyone. In addition, the question is almost always phrased as if they need to justify their decision to not have children. I understand why the potential grandparents might feel compelled to inquire, but some guy at the office? Really?
I do my bit for the married couples in my life by never asking. I was talking to a friend the other day, who complained how people tend to ask him when he's going to have kids, impatient for them to happen soon. I told him that I thought it was weird how people always asked these kinds of questions, and to the extent I had any thoughts on the matter, I was quite happy that they seemed content to not have kids in a raging hurry, because if they did it would really put a dent in our hanging out time. Even that, I said only because
a) he'd know I was kidding -I didn't actually consider it any of my business, and would be delighted with whatever choice they made, and
b) he'd possibly find it a welcome counterpoint to the pro-child chiding he receives too much of.
I like people having children. I also like people making their own choices free from needless nosiness.
You wouldn't ask a couple "how's your marriage holding up?". You wouldn't ask "how's that embarrassing medical condition of yours?". So why ask about something that has a good chance of bearing upon both of these?