It is interesting what you say about telling the difference between people who ask unbiased, just out of healthy curiosity, and those who will pick on you when you tell them you won't have children ever.
I think it works like that in every field. I have been a vegetarian for 12 years, and a lot of people(if not everybody) asks you why when they first know. At first I would come up with dozens of good reasons, from animal rights to environment, but a lot of people didn't ask me to really know, they just wanted to know which was the line they had to use to ridicule me. I am older now, and as with the children question, I think you have to really learn to tell the difference between honest unbiased questions asked by people who are open to really listen to answers, and the questions asked by people who hate the fact that there are people out there different to them.
I am convinced that people who react almost aggressive towards vegetarians feel judged in their lifestyle, like people who react aggressive towards childfree feel too.
You can take this comparisons to many fields in life. Some people don't accept that we human beings are all different, unique individuals. These people would like everybody to share their values and they feel uncomfortable when somebody is different (childfree, homosexuals, foreigners (or even vegetarians!) Some people hate the fact that there are minorities, and they are not worth an argument. A short answer and let's get them out of our way.
Sorry I rambled!