This little trio of Xs has multiple meanings and its translation is all about context. One of the oldest is for moonshine, booze, what have you. It's often seen on the sides of corked jugs and the like to let you know that it's knock your d**k in the dirt alcohol.
Next up we have the obvious pornography aspect, XXX is a rating that was applied to only the filthiest of porn movies back in the day. This also sometimes included excessive bloodshed or violence without sex. The rating system started with X, then double X, and of course followed by the aforementioned. It is pretty much a run of the mill label now.
More recently, it has been associated with the straight-edge movement. This movement represents purity of self, in one form or another, generally relating to abstaining from alcohol, drugs, promiscuous sex, meat, tobacco, or any other thing deemed impure by whichever group you talk to. So we would guess that the irony in symbol choice is intentional.