Its steam though and not german laws (not entriely). Its perfectly legal to buy porn games in germany. The only requirement is that a site requires a system that can legally binding prove that the buyer is 18 years or older.It´s not the German government banning porn ... it´s a (non-government) German institution demanding Steam to regulate access to porn games ...
No problem to get games of violence and gore and even Nazi-Themes on Steam ... but porn ...
OMG PORN ... nope ... no way ... never ever ...
*SIGH*
Steam is just too lazy and to cheap to implement such a system so steam just blocks germans from buying these games.