I know a few couples that don't have children by choice. In small talk, they are constantly being asked how their children are doing, do they have any children, are they planning on having kids, etc.

Even when it's not meant to be judgmental, those kinds of questions can get old after a while. Then, add in the fact that the line of questioning often is judgmental and you'll frustrate even the most patient person.

It doesn't take an overly sensitive person to get frustrated by that. Hell, I got frustrated by the attitudes and questions, and I actually wanted (and now have) kids.