Forums specifically for childfree people always have parents talking about how amazing their life is now, they've never loved so much, they're no longer selfish, childfree people will die lonely, aren't real women, will never find love, will die old and alone and they (the parents) will laugh, childfree people are materialistic and only care about money, they should only live in one bedroom flats and on and on.
Forums for childless people are full of parents telling them 'I struggled for two months to get pregnant, one night I was tipsy and relaxed and got pregnant, have you tried just relaxing?'.
Forums for parents who regret having kids are full of parent telling them they're disgusting and they love their kids and so do all normal people so there must be something wrong with them.
When someone asks what iPhone to get, people pile on about how they should get an android. Happens the other way around too.
People ask advice about a vegan diet and people pile on and tell them they need meat. People ask how much others spend on alcohol and people tell them they never drink and how they shouldn't either.
Basically a lot of people go to places that are the opposite of who they are and give their opinion. Welcome to the Internet!
Also lots of childfree and childless people on MN are teachers, doctors, nurses, specialists, whatever and so might be able to advise on children. Like supernanny, no kids but loads of parents want her help!