Games with explicit adult content are not available in Steam store, that is why devs who wants German customers hand out Steam keys or sell it also on other platforms (like itch.io). I hope the dev will also sell it here.
I’m just curious (and can’t buy a key on g2a or the game on steam), so don’t feel pressed to answer my ideas but i wanted to share them with you. Is it possible to switch the Steam API Integration off for non-steam-sales e.g. if not installed in a steam directory then don’t activate the Steam API? Another way would be to have a patron, steady or adult subscribestar to sell steam keys.