What sort of acts are we talking here?

I didn't want to be explicit, but you thought of the obvious example.

I can't think of any other examples, though.

probably not as bad as they could be if designed with that in mind (as opposed to, y'know, the enjoyment of the player.)

For the sort of 4chan people Eliezer mentioned, these would be completely congruent.

... maaaybe. Again, I'm not sure exactly what you have in mind.

I've never heard of such a thing actually existing.

Well would you, if it was grossly illegal to describe the contents?

It is well known that illegal pornography exists on non-interactive media. For interactive media, all I've ever heard of is 18-rated sex scenes.

Good point. Indeed, it's well known that child porn exists on some level.

In fact ... I do vaguely recall something about a Japanese game about rape causing a moral panic of some kind, so ...

EDIT: In fact, it featured kids too! RapeLay. It's ... fairly horrible, although I think someone with the goal of pure horribleness would do .. better? Worse? Whatever.