I've always been confused by these conversations though. Aren't people who are having kids doing so because they want to, whatever want might mean to them? Fulfilling just seems like another way to pursue fulfillment/happiness or whatever it is that individuals pursue.
When my wife and I chose to have kids, we enjoyed it. We derive fulfillment and satisfaction out of raising kids. Yeah it's frustrating at times, and you do have trade-offs, but we did it because we wanted to, to feel happy/fulfilled. We didn't start a lifelong journey to support children into adulthood out of some weird sense of patriotism or something. Anyone doing that is weird.