Look, I understand that it was a nudy picture. But one of the Stache has something like "Banging hendy's mom" writen as his location! If that is allowed (and it should be because freedom of speach means that you can say ANYTHING, not just that you can say things that are allowed ) then what's wrong with a link to a nudy picture. I mean, I would understand if it were real porn or something like that, but from what you and others have wrote, it was some kind of a joke, right?
The answer is same for all the adult sites on the web: You don't.
You just put a "You must be 18 or older to view this content" above your sign in. Besides, as that guy from "The Scrubs" said: "If the ever ban internet pornography there will be only one site left on the internet: the petition to bring back pornography."