When people date for a long time, people ask: When are you getting engaged?
Once engaged, people ask: When are you getting married?
Once married, people ask: When are you buying a house? and When are you having children?
So, you see, it is just a seemingly natural question people ask. Like, if you were in college, you might be asked: When are you graduating?
I suppose if one is offended by the question: When are you having children? They may think it was a rude question, while other people may not be offended whatsoever.
When people ask me questions that I do not want to answer, I tend to use humor as an answer as not to be rude, but not to answer the question, either.
For instance: When someone asks "How much did you pay for that?" I typically say "oh, somewhere between $3.99 and $900.00." LOL