I'm confused by your definition of "freedom of speech". You have the freedom to create whatever art/game you want. The dev could have created the game if he wanted to. Freedom of speech does not require Steam to host your game on their platform, however.
Not to mention, if you are a huge champion of "freedom of speech," why are you so upset that the community is "bullying" (your words not mine) when they are also expressing their views through their freedom of speech? So the game dev is protected by the freedom of speech but not the community expressing dissenting views?
Yes, Steam can decide to not host something. They have strict guidelines and quality standards. Which means the main content must have been in the game when they scrutinized it. And they did greenlight the game in the publishing process. Think about that. A professional or more had an actual look at the game and decided it was ok for the adult section of the platform.
Makes you wonder. Are those people greenlighting games idiots? Is Steam ok with "rape simulators"? Or maybe the activists trying to remove the game did not use quite the truth to rile up their angry mob... (Just read the statement of the developers what actually happened on Steam. Don't take our word for it).