They're ignoring the fact that some people can have their own children, love them and feel comfortable around them, and still feel awkward around anyone else's kids. If having your own kids isn't the magical trigger to make you interact easily with kids in general, I don't know what else you expect to do it.
Probably the worst thing is that when a parent of a new baby subscribes to this idea (that all non-baby people are just converts-in-waiting), they always seem to believe that their particular baby is going to be the trigger. Even if you tell them the last baby you held didn't stop screaming for a week and eventually had to be committed to a mental institution, they dismiss your protests and hand you the baby with a knowing smile, saying, "It'll be fine." Because regardless of whatever happened with other people's meaningless kids, nobody could hold little Jordan and look into his face and not have their heart melt.
I can't judge them for that. When I have a kid, I'm sure I'll believe that he is going to be president and a billionaire and cure cancer and fulfill some prophecy, and that anyone looking at him should be able to see it. That's just being a good parent.
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But it does make for a touchy situation for the non-baby person, because that's setting up some really high expectations. If you screw up and the baby starts crying, or you and the baby end up staring at each other in mutual silent terror as a tumbleweed rolls by, that kind of parent is going to be disappointed and might get a bad impression of you.
Again, I'm not talking about all parents, just the ones who are convinced that all people are really baby people deep down and that their baby can bring it out of anybody. In their disappointment, they might see you as a robot or a baby hater, which brings me to ...