I've seen that if I paid it on itch.io, I would get a Steam Key. The problem is that in Germany and some other countries, the users can't access or even view 18+ games on Steam, much less play them. What can be done except buying a VPN service as well?

Unfortunately we can't do anything about making our game available on Steam in Germany, so you'll have to use a VPN or simply play the itch.io version.

Sorry about that.