MarkoH01: You can acceess "bundesanzeiger.de" to see which games are on the index and therefore the games you can't sell to Germans without proper age verification ... well, you aren't even allowed to advertize them so this thread alone would be forbidden as well.
The main problem here isn't that these games are on the index or even that German law makes buying and selling AO games harder - it's that stores like Steam and GOG try to avoid the law by just preemptively blanket blocking anything that they THINK might fall under those laws.
As I've pointed out above, there's a process you have to go through in order to even determine whether your unrated game is considered to fall under these youth protection laws. Until your game has gone through that process (and some games never actually get reviewed in that way), it's to be treated as "unrated" - and to come back to my example from above: Dusk is unrated and it's still available, no problem.
MarkoH01: Nope, we are talking about actual professional porn sites with all the legal rules that apply to them (so nothing illegal). Yes, they simply said that it would vbe far too easy to access porn pics. If it would be up to them half of the internet would have been censored.
The thing with porn sites is this: as long as they aren't officially a company operating in Germany, puritans over here can't force them to shut down or implement an age verification system in line with German law. However, we've seen a
renewed discussion about blocking such sites via DNS blocks in Germany, which is arguably worse.
EDIT: Just to make a point about Steam/GOG preemptively blocking stuff: The Gamedec artbook is actually region-locked on Steam because the devs have declared it adult only and anything with that tag is automatically blocked. But from what I've seen, there's nothing in that artbook which would justify even a USK18 rating.