I completely agree that it is wrong to ask someone when they are going to have children or when they are going to make any other personal decision in their lives. It's just plain rude, but that doesn't seem to stop a lot of people. Maybe instead of "I'm sterile," you could say "I'm unable to have children," which is closer to the truth, since you are just not able to believe it's a good choice for you.

A lot of people define themselves by the fact that they are parents, and the question "When are you going to have children" probably translates - most of the time - to "when are you going to be like us?"

I probably would have had children if I'd ever seen any happy parents, but I never did.