The original use of "XXX" to mark alcohol was by a servant of the pope who he asked to travel ahead of him and mark taverns and vinyards that had particularly good wine. Later it was used to mark the number of distillations whiskey had gone through. I've never heard of it being used to mark poison. In the case of porn afaik it's supposed to indicate censored content. Edit: Looked it up, it's an MPAA thing, not something the porn industry came up with. This isn't intended as an endorsement of porn, but that meme is pretty boomery.