It really bugs me when childfree people say things like "I was born without the mommy gene." It implies that we're defective in some way because we don't want kids, or that mothers are somehow superior. I can only speak for myself, but I can tell you without a shadow of a doubt, I am not defective or inferior.
I also can't stand it when people say things like "I wouldn't be a good mother" just to placate mothers. Why make excuses? We just don't want to have kids. Making excuses makes it seem like there's something inherently wrong with not wanting kids and we need to make up some seemingly legitimate reason why it's not a good idea for us to have them.
If someone doesn't want to own a dog, they don't have to make excuses like "I was born without the 'dog lovers' gene". If someone doesn't want to climb Mount Everest, they don't need to make excuses like "I was born without the 'mountain climbers' gene" so why should we make excuses like "I was born without the 'mommy gene'"? It's a choice like everything else we do in life.
I don't care if someone has a problem with my not wanting to have kids. It's my life and my uterus, not theirs. They don't get a say in whether or not I have kids. My not having kids doesn't impact their life in any way whatsoever.