If you ask me it's more a case of what is generally accepted as a depiction of sex, versus what begins to be classed as pornographic. As this is the internet, of course someone has compiled all of the sex scenes from The Witcher into some sort of movie that has over a million views (because of course it does)
https://www.youtube.com/watch?v=MoH0ynsgccU (I would embed but the thumbnail will probably get me warned)
What you'll find is very little penis if any at all, a lack of vagina and mostly just breasts and implied sex (often with camera cuts and moaning). Implied sex meaning, yes, sex is obviously happening, but it's not graphic penetration. Something that TV and films can't get away with either or they'd be rated as AO/Porn.
Compare that to some of the stuff on steam which is far more graphic.
This starts to move away from the children/minor content and tries to understand why Steam might be hitting some games that don't have minors. Seeing as we already have a few "but what about TW3 and GTA?!" replies already.