I don't see why that would be the case. It's reasonable to allow services that have a policy forbidding such content and make good faith efforts to moderate and remove it promptly. Seems analogous to e.g. a building being vandalized with lewd drawings. Or laws about user submitted child pornography.

I expect most forums or discussion groups in practice actually don't have child-inappropriate content, and already moderate such things because the members don't want it.