I guess it's more about a face to face "confrontation" about differences and what that implies that can make us uncomfortable. When you're talking one on one w/a mom about not wanting to have kids there may be a perceived implication that because you haven't made the same choice that you dissaprove of hers.
I don't think it's limited to the CF issue though. I have experienced the same discomfort or feeling of subtle confrontation when discussing being a vegetarian w/carnivores and being an agnostic w/religious people.
It's more about two people who have made different choices looking each other in the face and dealing w/it. Can be a little weird at times, but hey, it's only a difference in opinion/choices. That's what makes the world go 'round.