Personally, I think these forums already tend to have a 12 rating in terms of language often used. There's ways of speaking about the topics you've discussed in a mature, adult way, and would be acceptable in an open forum imo on that basis. If people are worried about children accessing it, personally I subscribe to the notion said parents should be taking a much more hands on attitude to reviewing what said children are browsing, how and why.
My biggest issue is people taking these subjects in a not mature and instead very much 5 edgy manner where the point is to offend rather then constructively discuss these topics. But hey that's why we have mods right? So, I guess I'm split on even having these topics here in the first place.
On the topic of open forums vs a private members only club, honestly I feel the open nature of this forum is woven into its fabric for better or worse. Hell, moderated posts almost never get deleted and instead sit in katakombs for everyone to see, defeating the point of moderating it. But that's just part of this forum's culture.
In review I think tentatively I'd say no to private members section, as I fear it'd encourage cliquism that's already been accused of on these very forums (despite all content being open to public). As for the subjects at hand, I'll say I'd be ok with a mature, adult and serious discussion of them on open forums, but I can already see the trouble it can bring when the discussion turns very much insenstive, immature and edgy for the sake of edge. But I'm biased like that.