The problem here, as many have already stated, is that this is a very subjective proposal. And it is very hard to codify a rule based on a subjective. What you consider vulgar or obscene my not be close to crossing the line for me. Rules need to be objective in nature. They need to have the ability to have clear set guidelines that mean the same thing to everyone.

It is similar to the old argument about porn. What is porn? I've seen artistic photographs of nudity and I have seen porn. But where is that line drawn? That line is going to be somewhere different depending on the person. There are those that would say any photograph or video where the subject is nude is pornographic. Some would say only if there is a depiction of a sexual act, it is pornographic. And there are a lot of people that fall between those two and go further to the extremes in either direction. In that case, you cannot easily write an objective rule that everyone will interpret the same way. You could, say, write a rule about not having any depictions of nudity. That is more objective. Still not perfect...because what if it's just one breast but the subject is otherwise clothed?

Anyway, this is getting way more wordy than I intended. The TL;DR is that trying to codify a subjective rule is both difficult and a really bad idea because no two people will interpret the rule in the same way. We need to focus on objective rules that leave little-to-no room for misinterpretation and solve or prevent actual problems.

But this is a great discussion on civility guidelines and I do think we need some of those. Though, again, they will likely have to be somewhat vague because of the subjectiveness of it all.