Valve does have policies, content ratings, and content filters which (supposedly) prevent anyone from "innocently" purchasing/installing the title. They have to be registered as adults. They have to "understand and agree" to notices and warnings about explicit material, etc.

I'm not as shocked as you are. Every other online media provider offers adult materials, games are just another form of entertainment which makes money. I don't know about anime schoolgirl stuff like this, but I do know that other games tend to sell better if Lara Croft wears skimpier outfits or if Dogmeat takes down opponents with more blood and gore and savagery. Adults buy games for their children. But adults also buy games for adults.

This game might be porn. But it's not "uncensored" porn if it can't be viewed or accessed by those who shouldn't (or who don't want) to see it.
Its presence in Valve's product line doesn't offend me as much as Valve's failure to keep it "censored" away from the wrong audiences would offend me. I hope it is properly "censored" since it does (at a glance, in a thumbnail) look like the sort of thing a child might want to click on.