porn is prostitution, sex in exchange for money, which is illegal (at least in the US)
Uhm. Sex in exchange for money. So ... marriage is also illegal? There must be money involved, otherwise there would not be payments after a divorce. Money is involved in there and refusing to have sex is grounds for a divorce.
Porn is not prostituion. Prostituion is a service that involves actual sexual services (in the flesh, literally). Porn is short for pornography, which is the depiction of sex.
We could even argue about, if depiction of non real people does even qualify for being porn. It is artificial pixels. Drawings. Renderings. Not real. It is not porn, it is a drawing of porn. Same as porn is not sex, it is a depiction of sex. Porn is once removed from the deed. Cartoon porn is twice removed from the deed. And prosition is the deed (with the special case that it is in exchange for money).
If porn is speech
It trivally is not. But in that one country where free speech is a thing, it is considered so. There is no dispute about that. But that one country also has those ridiculous obscenity laws which circumvent the free speech by very vague and arbitrary conditions.
Do you actually think it was illegal for them to remove this game?
Legality does not come into this. A platform can remove anything for any or no reason.
It is about how a group imposes their agenda by bullying, threats, lies and other questionable methods. Platforms rather take a thing down than deal with that.
I do not believe it is good for society to listen to such emotional manipulators. They rile up an angry mob and steer it. The game itself is not illegal*. The content might be disgusting to many. So what. Taste is individual and what one adult views as entertaining is not the business of another adult. What fictional things are "allowed" and which are not should not be subject to the moods of angry people. If we want to have free speech and things like that, we must allow all of it. Not only the things we deem appropriate.
*There is dispute worldwide what constitutes illegal. It gets ridiculous if you step back and remember that it is fiction and some fictional things are illegal in some countries and legal in most others. Even adult games themselves are illegal in some places.