Unfortunately a lot of times it's not a joke. These people genuinely think they're superior to parents, and a lot of them genuinely hate kids and those who chose to have them. It's a rotten mindset to it's core that built on hatred of preconceived stereotypes. It's something that's irrational in both logic and the emotions that it evokes. It's literally a new form of bigotry.
People like you should be the default. You made your choice and you respect other people who made theirs. You understand other people have their own reasons that are different than yours. That's normal, that's healthy. It means you're secure enough in the decisions you've made to not go around trying to justify it to yourself by pretending you're better than other people. As much as I would like to believe that people like you are the silent majority, I'm finding that more and more difficult to believe with just how prevalent these smug childless people are becoming in society.