People who don't conform threaten the cultural standards of society. By being child free you suggest that it's an equal option to having children. (Which it is, but bear with me.) People who comform willingly see you as misguided typically, depending on how far down the "True believer" spectrum they are. This is your "you'll understand when you have kids" sort. People who feel pressured into conforming will IME be more likely to attack you directly: "You're just selfish." Because if they accept that you have an option, they have to accept that they had an option too. There are also people who will see child-free people as judgmental of "breeders." (Which of course, some are.) So there's a preemptive jump to defend themselves or attack rather than have to defend.
Replace all that with vegetarianism, polyamory, atheism, teetotalers, etc. Even people who don't smoke weed will get crap if their friends do because they're "judging" them or "think they're better than" them. And there are absolutely some judgmental jerk veg*ns, poly people, atheists, etc. out there. So just as a bad experience with a religious fundamentalist might make me defensive about my pansexuality, a bad experience with a "YOU HAVE BLOOD ON YOUR HANDS" veg*n might instigate defensiveness.
Like most things, exposure to different beliefs reduces this freak out and creates acceptance.
(As a side note, I was never using a**hat as a way to dodge the filter, but I'm still sad it's now filtered. le sigh, I just like it!)