I've heard that the tattooing is actually done not with needles but their own dicks. Dude tattooing a tattoo of himself getting a tattoo of another dude tattooing his dick on another dude's dick with his own dick. Then vice versa. Some postulate that this is a formalized courtship ritual, but much is still unknown.
Apparently this sort of behavior actually has a small but rabid contingent of adherents. It was born from tattoo-obsessed heretics of the "Docking" and "Frottage-ing" schools of dick on dick action that I suspect are also at least partly, perhaps subconsciously, motivated by a Matryoshka doll mommy obsession.
I think Vice is working on a documentary. Hard hitting journalism type stuff. Like such (NSFW probably)