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I don't know, but there must be some reason traditional imageboard design mandates a seperation between sfw and nsfw. Do you really want to leap that fence? What horrors lurk on the other side? Chesterton is frowning at you.
My best guess is that porn is distracting. It lessens the capacity for intellectual activity. You don't jerk off where you work - gotta keep things compartmentalised. You can't have a good discussion about literature (or traditional games, or weapons, or papercraft, or comics and cartoons, or animals and nature, or, say, the art of rationality) if the temptation to succumb to your base impulses is all around.
Having rules also has the meta-benefit of creating a community through shared norms. We can delineate between good posters and the hated outgroup based on their behaviour. That said, I don't think rules beyond basic prohibition of informational/legal hazards are necessary here. We're few enough, obscure enough, and self-selected enough that we repel outsiders natually and all have enough in common to form a primitive community.