I wouldn't recommend doing it to strangers - but isn't being all up in each others' business what friendship is all about?

I don't think the solution is to not ask anyone any personal questions ever. But how questions are asked is often more important.

"Why haven't you had children yet?" or "When are you having children?" - These questions both carry some pretty serious subtext with them. They both imply that there is something wrong with couples that don't have children. Probably not what the Asker was intending, but still the implied connotation. Granted, some people definitely do think there is something wrong with couples not having children.

"Have you guys talked about having children" is a more open ended way of asking the question. It allows the Askee to answer in a way they are comfortable - "No we haven't" shuts that shit down pretty quick. As it is non-judgmental, it allows for an honest answer - "We have. We'd like children but can't have them" or "We have. We're going to wait a few years until we get better jobs" or "we're actively trying right now. Please leave our bedroom." It gives people the opportunity to respond in as vague or as accurate an answer as they feel is appropriate without implying there is a correct way to answer the question - because there isn't.