People who have kids have to put a bunch of their resources towards those kids: money, time, lost sleep, lost interactions with friends, and so on on. Nothing controversial here so far. Some view this as a choice of those people because they simply want to have kids, raise a family, and so on. But regardless of their intentions, there is a positive outcome for society in that there now exists a next generation that will keep society functioning. Those kids will grow up to be the doctors and farmers and so on that do all the things that need doing. These kids really are an investment in the future. People who don't have kids, but still grow old and live in society, say retired on purpose, or simply because their bodies and minds degraded to the point they can no longer work (happens to everybody), then those people are living in society that is functioning because of the time/money/resource investments of the people who had kids. The people who didn't have kids got a free ride because they didn't have to do any of that, but still get to see food in the grocery store that go there because other people had kids who then grew up and made that happen.
It's fine if you don't want to have kids, but if you plan on being alive and in society in 20-30 years, then you are living off the investments of others. So why not let them have a bit more salary then you?