It's the general tone of disgust thrown at them usually.

Nothing wrong with not wanting children, but a lot of these people act disgusted, like your kids are the most disgusting thing in the world, second only to you for having them in the first place.

I'm a single parent. I've been on the receiving end of some that back when I was dating. People can say it's not personal, but when you act like that it's just bs, because how is that not personal? People don't usually think very well of people who act so disgusted over height, weight, race preferences for good reason. It's a shitty way to treat someone. Yet for some reason, it's somehow okay to don't exact same thing to someone for having had a child. Make it make sense.

Maybe this is some kind of blowback from overall societal pressure on single to have children. They've been harassed about it so much they don't realize they're acting disgusted like they are. Face it, most staunch child free people are pretty adamant, aggressively absolute, and forceful about rejecting the idea whenever it is brought up.