I would say that the forum is simply reflecting the rules of real-world establishments of a similar nature: It certainly doesn't bring anything new to the table in that regard.
Now, regarding your points. A gun does not offend or kill when you look at it. It's nonviolent aspects are able to be used in other ways that are culturally acceptable. You can write a wonderful story and include guns in it, for instance. You can use guns to criticise existing cultural norms, as you've done here. A gun is not obscene. The usage of guns can be. So its potential obscenity only comes into play when the culturally unacceptable behaivour (i.e shooting someone with it) occurs or has the risk of occuring. This means that you can not threaten to shoot someone (provided you truly have the means to do so, and are serious, both aspects that can't be proven easily) or put up a video here where you film a shooting of any person (arguably any other living organism).
Pornography has less consequences attached to it, because it is as you say: Everyone enjoys porn on some level. What is so offensive about it? It's basically an in-your-face demonstration of something some people deem should remain private. Does it remain private? No, not at all, just watch TV, it's basically the same thing. There's a reason why most of the actresses you see are morphologically attractive women. TV programming gently teases; porn gives you the whole thing. Some people are shocked by that. I don't know, don't ask me; what I know is that we still need some time before pornography is accepted on the same level (and I'm talking about North America here: I live in a sucky country where this will never, ever happen and discussing stuff like this lands you in risk)
Now, the MBTI cult is not a culturally forced phenomenon. You can, for instance, join this forum knowing nothing about MBTI, and under the guise of a mild interest, proceed to never discuss it again while continuing to participate within the sub sections. This is where this internet forum truly becomes an internet forum: It emancipates itself from cultural norms and becomes a flexible entity with the ability to accomodate multiple viewpoints. That's the idealized essence of an internet forum, a highway of ideas: It's one of the reasons why 4chan is so successful (or unsuccessful). But idealized essences almost never become real; somewhere down the road to realization, we simplify and normalize the idea into something familiar, because we're all fond of the familiar.