It is true that people can be total assholes when it comes to having/not having kids. Asking someone why they don’t have a baby, or slipping them the number to a fertility doctor, is not polite! And I agree that more people should actually think through whether or not they want children; it would also be nice if we lived in a society where having children wasn’t assumed to be the Best Normal Thing once you hit a certain age and marital status.
But you know what rudeness toward child-free is not? It is not “almost the last remaining prejudice.” It really really is not.