I really didn't realize that having children was a choice until I was in my mid-20s. I mean, intellectually I knew it was a choice, but I had still always just assumed that I would have them someday. Who doesn't? And why wouldn't I?
Then all of a sudden I got married and started my first post-law-school job, and realized that "someday" wasn't so far off anymore. And just a few months after that, I came to my decision that I didn't want them. Ever.
Thinking I'd have kids "someday" was easy -- thinking about what they'd actually do to my life now, or at some point soon over the next few years, isn't. "Someday" is very different when you're a child yourself than it is at 26. All of a sudden I realized that that mythical "someday" when I was actually going to want a child was never going to come.
And I had never really wanted it to. The whole thought of "someday" in the first place was socially conditioned. It never really came from what I actually wanted as a person. I suspect many childfree women have gone through the same thing.
Once I stopped thinking "someday" and started thinking "This is my decision," everything suddenly looked very different. And it never looked the same again.