Processions with huge phalloi were the most public form of Dionysian worship. The priapic was a way to honor Dionysus and even to represent him. Since (as Heraclitus says) Dionysus and Hades are the same, the phallos triumphant belongs as well to Hades, and therefore has a subtext, an intention below what is blatantly exhibited.
Something mysterious goes on during arousal. Since pornography aims at arousal, it must also have an underworld meaning of mystery and not merely an underworld meaning of criminality. Or, perhaps we must accept that both underworlds go together—the mysterious and the criminal.
The very subject, pornography, let alone its images, disturbs our conventional reason because pornography invites both Dionysian ecstatic vitality and Hades’s rape and death. The erected phallos acts as a fascinating emblem of both desire and terror.
— Pink Madness, or Why Does Aphrodite Drive Men Crazy With Pornography?, Mythic Figures, UE6