The other common (valid) argument is that it's easy for children to access this content because they're already using Steam and unless they have mature content filters enabled, it's already trivial to bypass age gates by lying.

That said, I don't agree with censorship and especially by payment processors of all groups. The slippery slope is very concerning for adults who would enjoy any other category of content that are targeted by activist groups. Collective Shout has a history attacking media falling outside the porn bubble.