Steam would be liable only if illegal content gets published, not uploaded, right? They'd have to implement a verification process for uploaded content, and if nothing illegal slips through their net, they're golden - right? It'd be nonsensical to make a company responsible for the shit people upload. This is an important distinction. If you don't have this, any kind of website form would make running the site legally impossible.
The law is very precise and clear (for the most part) when it comes to what's allowed in terms of adult content, and what not. It'd pose no problems for say Rockstar to continue making adult themed games. Just like it already is in film, literature and other forms of art right now. I don't see a problem here.
These initiatives are probably meant to put pressure on companies like MindGeek, which have proven to not care about the legality of the content they're hosting. This is not about censorship but about straight up crimes on video being published by these people. I can't see why anyone wouldn't want this practice to stop, because obviously what we're doing now isn't working, and it's causing real harm to human beings all around the world.
I don't like censorship and moral policing just as much as the next guy, but when bad actors can hide behind the constant cries for "censorship" and the like so easily, there's clearly something wrong. I swear some people in the gaming community are protecting real criminals with their hysterical behaviour. And yes I do understand that for historical reasons, you Americans are very sensitive to that topic, and that's not a bad thing. But let me tell you here in Europe we don't use funny names like Karen for this, because here actual, real state censorship was a thing, and in the collective memory of Europeans it is linked to totalitarian regimes that caused the death of millions of people.