Take the dismissiveness and shove it up your baby hole.
When you say “You should have children!” or any of its variations, such as “It’s different when they’re your own,” and “You’ll change your mind someday,” we hear something totally different. What we hear is “I have no respect for you. I never did. I wish a bleak and horrible existence upon you. In fact, I wish you to live in stifled misery akin to the most brutal and relentless slavery until the very hour of your death.”
When we say “I don’t plan on having children,” what we mean is “Ever.” Not “Maybe.” Not “Someday.” It is “Ever.” Period.
Whether we will change our minds someday or not is irrelevant. If we said that sort of thing while you were pregnant, you would never forgive us. Especially if we gave you coat hangers for the baby shower. Just in case.