It's not Steam that isn't allowing it. It's the German government who demanded that Valve bow down before them by implementing a bunch of changes to Steam that Valve didn't want to implement so they just blocked all 18+ erotic games.
Since Steam only blocks access to the steam pages of those games on Steam if you've set your region to Germany, they don't actually make it impossible to own those games. It still works when activating a steam key as long as the steam key is global, some regions are also capable of gifting the game to a German steam account.
So you don't have to worry about Steam not allowing it. Their solution was actually pretty lazy all things considered. I think it's because Gabe Newell did not care much about or for the demands of the German government.
So that only leaves your worries about me reselling the key. I can only ask that you trust me as a loyal customer, I kinda bought your game because I was hoping for a steam key. I like all my games in one place if you get me? Adding Benefitship as a non-Steam key is a constant reminder that my government seems to think it gets to decide the content I view in my own home despite the content only featuring FICTIONAL characters (who are clearly depicted as ADULTS who CONSENT btw). And that feeling bothers me a lot. I'm in my late twenties, not a teenager who needs a babysitter ffs. Pretty please reconsider? I'll provide proof of purchase...