Maybe some people actually think we can't have children, so we just tell people "we don't want them" because we don't want people feeling bad for us.
A few years after we were married, my step mother (whom I'm not very close to, and was probably just trying to make conversation) asked me while we alone in the kitchen, "So, when are you 2 going to have kids?" Not someone I should deflect with witty one-liners, I figured now is as good a time to get the info out and she can rehash it to my Dad later. So, I replied, "Actually, we have decided not to have children." Literally, her response back to me was, "Well, have you even tried?" For a second I thought maybe she misheard the question, so I said, "No, I said we decided not to have children. She said, "Yes, I heard, but have you guys even tried?" I'm sure you can all imagine my confusion. I just said, "No, we haven't tried, because we don't want them." She said back, "Do you even know if you can get pregnant?" What does one say to that? I was like 24 years old. All I could say was, "I assume I'm capable of reproduction, but I really don't care seeing as how I don't want children." Then I pretty much just wandered away.
It was a very weird conversation. I get the impression she just thought I was putting on a brave face, you know. "Pretending" I don't want something I can't have. Maybe they think those of us without kids are constantly wondering about our fertility "just in case?" Is it something all women who actually want kids wonder about until they actually try to have a baby? Wow, it probably is, isn't it? I just spent all my time trying to avoid pregnancy, never cared if I was capable of it.